My boss is a very good guy, but he has somehow promised to re-classify my current position ( to higher level) already. If he learnt I wanted to move somewhere else, he may change his mind and not re-classify my position any more.
PS, if I'm leaving, it's going to take him a good while to find a person and wait till that person become fully functional. So he won't be too happy to know this even though he is a very good person.
1. everyone is replaceable
comefromwhere is right: you do not need to worry about your boss so much. 2-3 weeks notice is enough, and you can do some part-time job for him after you leave.