one user's BB is activated with one mail address. but if he want to change mail address & mail server, like from Domino to Exchange, what should user & BES admin do to re-activate BB with new mail address & new mail server?

if your BES servers are in the SAME BlackBerry Domain (meaning sharing the same BESMgmt database), just do a user move and background EA should take care of the service book update. Email address information is stored in the Desktop [CMIME] service book. Otherwise, you would need to remove and then add the user to the new BES.

I missed the keyword different platform. You got to do the user move between the mail platform first. Then remove user for source BES. Wipe handheld. Finally add and activtate user on the destination BES.