Here's what I learned yesterday because I actually ended up with three situations involving changing names, merging, etc ...
Your watchlist follows your email address. So, if you need to merge your profile with one that someone else created for you, just make sure that you select the email checkbox during the merge so that your email gets attached to the new profile.
I still don't have confirmation on this, but I believe that merging to a new profile (including a Last Name at Birth change) will wipe out your contributions.
Can anyone confirm that?