Well, the thing about a princess running away (with the assumed era in mind) the princess would most likely have been dishonored and the name of princess would have been taken. Or they would've killed her if she did decide to come back, but that is the crueller punishment.

The bastard wouldn't have been named a princess unless there was no other heir to be coronated. In those cases, the bastard would step in (example is Elizabeth of the Tudor Era although she was technically not a bastard) and become a queen. 

The best thing to do is to just lower the age. Thus no conflicting issue on changing the bio or anything else. Those are just my thoughts, though.