I moved about 1800 users from a handful of LX controllers onto a pair of MXe Servers a couple years back. Those were all running release 9.0 at the time so we had to do it with OpsManager running SchedMAC's. I ended up breaking it up into 3 phases over a few weekends since we had to re-register each set after it was moved, so I'd do all the programming on Saturday and come back on Sunday with some helpers to re-register and test phones.
The single biggest headache I had was Ops failing to move the sets because of remote BLF's. It's also a good idea to export all of your set-specific data (keys, re-routes, station service, etc.) so that if it all goes south you can just blow away the set and rebuild it from your exported data. Also double check your resources & dimensions, particularly the stuff no one thinks about like the limit of 16 multicall appearances of a DN and Busy Lamp Groups.