the 2 commands you'll want to look at are
if it's the username you're looking to change you'll want to change the attribute zimbraDataSourceUsername . How can you do this for all or most of your users? well that'd take a bit of work. you'd basically need to loop through all users (use zmprov getAllAccounts), then do a getDataSource on each account, match the username to a regexp or something, then fetch the datasourceid and run the modifyDataSource on that.
Yeah not easy, but doable