It's quite simple to change the battery in the Mega 1000.
Jut remove the 4 screws, remove the old cell, and place the new one in. It's a CR1620 which isn't easy to find where I live. I just grabbed one online.
Be careful you don't lose the little piezo springs..........or the buzzer won't work
3/4 of the way down this page is a picture showing it with the back off:
http://forum.tz-uk.com/showthread.php?2 ... radio-sync
Regarding it not syncing, I can't help you. Perhaps they damaged the antenna when they changed the battery. They must've been pretty rough with it if that's the case.
Rgds.