Everyone here has some good advice, but I didn't see this one listed: I work at a place that just finished the migration for ONE customer (of many) and it took

Everyone here has some good advice, but I didn't see this one listed: I work at a place that just finished the migration for ONE customer (of many) and it took about 2 years. The main issue we ran into was that NOBODY documented the difference in IDs between the old vs. new system. We had to Frankenstein that shit (look at original filenames of imported data to deduce what new id matched to which old id) which took MONTHS. So, if you have any data, make sure you know EXACTLY what the id is in BOTH systems, even if you think they should be exactly the same. FWIW, our first step was keeping both systems synchronized (via that ID matching up) and migrating the end-user frontend to the new system. From there, we trained the customer on the new system and administration, and finally, we swapped them over to the new system and disabled the synchronization system. Now, we kinda know how to do it and we expect to be able to do it faster ... we'll see.