You could try using switch device wizard instead of a simple backup of one and restore to the other. I don't know if it will make any difference; but maybe worth a try if you didn't do it that way before.
FYI, the safe thing to do is store the contact in your address book with their PIN #. Put a notation in the notes that it is a BBM contact. Or create a catagory. Then if something like this happens in the future, search on the notation to find the BBM contacts again, or filter on the category, and add back to BBM.
When you eventually get a device on which to run the latest BBM, you'll be able to do a wireless backup as AK says.