I've spoken to Tech Support and referred to the website which explains the Data Conversion process that PCP will do for us. What I am concerned about is the turnaround time, and how the newly converted data is to be installed into our new PCP installation.
According to the website and Tech Support both (which incidentally IS consistent

What this means is that our church office operations (that involve the church management system) are to cease until the data conversion is complete.
Is there NOT a process in place for this very thing (migration from one system to another?) that does an ADD of the data to the data files, instead of a REPLACE?
It would make sense to cease entry into the now obsolete church management system the moment we do the backup. While PCP is doing the conversion, we then would be adding data to the NEW PCP system. And when the converted data files are received back from PCP, there would be an option that would do just what an "initial load" of data files would be...ADD the records.
Any ideas? I just can't see it as a feesable option to go totally "manual" for a two week period....