Not to belabor this ( I know, that is exactly what I am doing), but I hear the following: don't do a data transfer because the data is inaccurate, but oh by the way continue to use the very same data every single day in the extant system until you do change over.
I know many "data experts" recommend re-keying. I think there is a place for data transfer and a place for real time data verification (esp med lists -what a burden keeping those straight).