Using BIS on T-mobile (US) so it might not be the same to you. I frequently switch phones which include a Treo 650, T-mobilw Wing and some other small GSM phones. As long as the new phone is unlocked, you are good with phone and SMS. However, you need to put in right APN, ID and password to make the internet and MMS work.
There are quite a few threads on setting for different carriers. Roger should be one of them.
I can get emails instantly on both my treo and Wing (WM6) with the same SIM/phone number. However, 8700g wins when you need something clean and stable