Back in the day (I mean WAY back), the standard practice when switching from one database system to another was to run both systems in parallel. That is, you continue to run the legacy system even as the new system is brought online. Typically this meant double data entry (i.e., the same data had to be entered in both systems). Not only was it incredibly time consuming, but it was also rife with opportunity for error.
There were lots of reasons to run parallel systems back in the day, but most if not all of those reasons have disappeared with new and lower cost technology. With the implementation of a new AMS, you should be able to sufficiently test the new system before bringing it online. And once it’s fully tested, you should be ready to switch over to the new system without needing to rely on the old system for anything (other than perhaps looking at legacy data that you didn’t’ convert).
These days, if you’ve prepared properly, there is no need to run parallel systems.