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Have just been working on a cut over document. In this case it happens to be a migration from almost nil /manual system. So, we shed be able to do it with hard stop.
In the earlier migration from one rep to another, we used a hybrid method. Pick the cut over time - we chose the new fiscal.; pick the opening balances along with the open transactions with requisite masters to the new system; allow the users to contnue with the transactions pertaining only to the last period, while the transacted in the new system for all current. Waited for the stat audit to be over. Picked up the incremental balances and adjusted the new system accordingly. Carried from there. The challenge was / and probably is from the open trans. We really had to work hard in identifying all the open trans and we worked towards reducing their count all thru the implementation.
I think the worst choice is an implementation with a hard cutover. It used to be done, with the legacy system being cast into the dustbin when the ERP system was implemented. I suppose the pro is that the ERP system is installed in one fell swoop. The downside is that the system is installed in one fell swoop. No one is familiar with it, systems aren't optimized for it, and it may or may not work "out of the box". Bad call, in my humble. You can choose to proceed with a variation of this where you push in an ERP system to replace a specific process. The problem I have with this is that there is often little benefit over just sticking with the legacy system, particularly because there is no (or little) linking of systems between business units. On the upside, if things go wrong, there is little disruption to the wider business, and you can ease into the implentation...and keep easing into it for a long, long time.
Another option is that offered by SaaS ERP vendors where ERP can be rolled out bit by bit. This is easier in that it mitigates software/hardware installation issues compared to traditional installations and allows quick implementation, while still maintaining the legacy system outside of the specific system being replaced. A major benefit of this approach is up-front cost is substantially lowered, which is always welcome.