Parallel running. It sounds like the responsible, risk-averse choice. You're investing serious money in a new ERP system, so why not keep the old one ticking alongside it — just in case? It feels like a safety net, leadership likes it, and it practically writes itself into the project plan.
But in most cases, parallel running makes your go-live harder, increases your risk of failure, and is the opposite of safe. Here's why.
You No Longer Have a Single Source of Truth — You Have None
The most immediate problem with running two systems simultaneously is data. From the moment you go live, every transaction entered in the old system creates a version of reality that the new system doesn't know about. Reference numbers diverge. Automations in the old system fire when they shouldn't. Credit notes, invoices, and purchase orders accumulate across two platforms, and nobody really knows which one is right.
You haven't got one system that's true and one that isn't — you simply don't have any system that's true.
There's also the human reality to consider. People get interrupted. The phone rings. Someone asks a quick question. A transaction gets entered in the old system and never gets duplicated in the new one. And how exactly are you planning to track that? The honest answer is: you can't. The chaos compounds quickly, and the downstream impact on customers — wrong reference numbers, duplicate credit notes, conflicting invoices — can be very real.
You're Doubling the Workload at the Worst Possible Moment
Go-live is already the most demanding phase of any ERP project. Your team has spent months in training sessions, configuration reviews, and testing. They're rebuilding muscle memory, learning new screens, and keeping day-to-day business running at the same time.
Asking them to do everything twice — in two systems — at this exact moment is a fast track to burnout and failure. People will default to what they know. The old system quietly becomes the real system, and the new one struggles to gain traction.
If someone doesn't know where to find something in the new system on day one, the answer is not to go back to the old one. It's to stay in the new system, ask for help, and work through it. The crutch people need is good support — clear work instructions, an accessible implementation partner, managers who are equipped to guide their teams. Not a second system running in the background.
It Tells Your People That Leadership Doesn't Trust the New System
This is perhaps the most damaging consequence of parallel running, and the least talked about.
When leadership signals that the old system is staying on as a fallback, the message that lands with the wider business is not "we're being careful." It's "we're not sure this is going to work." Culture follows leadership. If the people at the top aren't fully committed to the new system, nobody else will be either.
That lack of confidence will sit in the background of every problem that arises at go-live. And when something does go wrong — and something always does, in every implementation — users will use the old system's continued existence as permission to retreat. It becomes a self-fulfilling prophecy. You were worried it wouldn't work, so you kept the old system running, so nobody ever fully committed to the new one, so it didn't work.
If you're going to make the investment, make it. Half-committing to an ERP migration is one of the more expensive ways to waste money in a business.
If Parallel Running Is Being Discussed, Ask Why
If the conversation about parallel running is even happening, it's worth pausing and asking what's driving it. Usually it comes down to fear — fear that something will go wrong, fear that the new system won't quite cut it, fear of a big-bang change affecting every department on the same day.
That fear is understandable, but it's also a signal. It might mean the project hasn't been tested thoroughly enough. It might mean the right stakeholders weren't involved early enough. It might mean the go/no-go process hasn't been rigorous. If you can't look at your leadership team with confidence and say "here are every show-stopping scenario we tested, here are the issues we found, here is how we fixed them, here is why I'm confident we're ready" — you probably shouldn't be going live yet.
The antidote to parallel running isn't bravado. It's thorough preparation, communicated clearly. Real-world test scenarios that reflect what actually happens in your business — the awkward custom order, the customer who wants it tomorrow, the factory that's already slammed — not tidy fictional ones. Evidence that the system has been stress-tested against the reality of your operations.
Get that right, and the parallel running conversation largely takes care of itself.
Adoption Doesn't Happen by Accident
Even with the old system switched off, users will default to what they know. Legacy Excel spreadsheets live in folder locations nobody told IT about. Old habits don't disappear on go-live day.
Sustaining adoption requires active effort. Continuing training sessions in the weeks and months after go-live — not just a hypercare period where people can drop in and ask questions, but structured, repeated reinforcement — makes a real difference. So does accountability. One approach I've seen work well is using the new system itself to surface data quality issues: if users can see when information is missing or incorrect, and that visibility is shared across the team, peer accountability kicks in in a way that no policy document can replicate.
Incentivising good practice early on also helps. People don't know what they're missing until they've experienced the new system running well. Once they've had a taste of clean data, reliable automations, and processes that actually flow — they won't want to let go of it. But you may need to give them enough support to reach that point before the intrinsic motivation kicks in.
It Delays the ROI You Sold the Project On
There's also the straightforward financial argument. Your old system costs money — licensing, maintenance, support. Every month it runs alongside the new one is a month you're paying for both. More than that, every month your people are splitting their time between two systems is a month the efficiency gains you promised leadership haven't materialised.
Quantifying that ROI properly is worth doing before go-live, not just after. Go into each department. Map the processes. Push people to put numbers to the time they're spending — they'll say it can't be done, but it can. Thirty minutes per transaction, twenty transactions a week: that's ten hours. Multiply it across a team, across a year, and suddenly you have a very clear picture of what the current way of working is costing you. That's the business case for the new system, and it's also the clearest argument for not delaying it by keeping the old one alive.
What About Going Live in Phases?
A related question that often comes up is whether to go live one department at a time — finance first, for example. For a business implementing a fully connected ERP covering warehouse, production, finance, and sales, this is usually impractical. Everything is too interconnected; you can't separate one module without creating a new set of bridging problems.
There are genuine exceptions. If a specific module has deeply messy data that genuinely can't be migrated yet, or if a business only needs one area of functionality to start, a carefully managed crawl-walk-run approach can work. But it should be the exception, driven by a specific problem — not the default template because change feels overwhelming. The fear of changing everything at once is understandable, but it's best addressed through better preparation and communication, not by fragmenting the go-live.
The Bottom Line
Parallel running feels like risk management. In reality, it is the risk. It undermines data integrity, exhausts your team, erodes confidence in the new system, and delays the return on a significant investment.
The better path is doing the hard work before go-live: thorough testing against real-world scenarios, honest stakeholder communication, and building the kind of evidence-based confidence that makes a clean cutover feel like the obvious choice. Go live once. Go live properly. And then don't look back.
