I'm not sure if rule is the right word but here is the direction from the help index about merging:
You will be invited to propose it, i.e. to create a Pending Merge. The appropriate Profile Managers will be e-mailed so that they can approve and complete the merge.
So it shouldn't be any different for a pre-1500 profile than any other profile is the point I was making for Paul. (What is a bit odd is that the merge seems to originally have been proposed January 11 yet shows todays date when a compare for merging is done).