For now, if you want to modify the definer of a trigger or any other of its parameters, you have to drop and recreate the trigger and manually lock/unlock the table to do this seamlessly.
Another "possible solution" is to manually modify the .TRG file (which is definitely not recommanded).

The ability to do an ALTER on a trigger to modify the definer, ENABLE/DISABLE the trigger, change its SQL mode and charset/collation would be interresting.

There has been some chat about ALTER trigger ENABLE/DISABLE for quite some time on MySQL : http://dev.mysql.com/worklog/task/?id=2825

ps: an ALTER VIEW DEFINER should also be implemented as the only alternative at the moment to do so is to modify manually the frm followed by a FLUSH TABLES which isnt a proper procedure.