I won't even bust your chops about the search....
Backup the old BB
Switch SIM cards
Log into BWC and change the PIN/IMEI number and resend service books
Restore the backup done in the first step
I've never called T-Mobile when I've swapped BBs.
Remember to wipe out the old BB when done.
You may have to contact vendors for registration codes since your BB has a new IMEI and that's what they normally base their security codes on.
But I'll leave my auto signature anyway