Check Out the BES manual (and a myriad of great posts herein) on the process known as a 'Knife Edge Cutover' for BES. I migrated 3 servers (2 in 5.5 and 1 in 2000) and a total of 1000 users with this process. It's VERY easy once you get the basics, and there are a lot of instructions and good references here that will save you alot of time, especially WRT manually re-adding users. Trust me, once you do it you'll really be happy and it will save you alot of headaches. Just read up and prepare and do your homework. Good Luck!