So what happens if I make Jack of All Trades a noble and then play Loyal Guard as a dupe and then at then end of the phase the jack of all trades becomes a non noble again? Nothing right? Since the noble is just a targeting restriction right? or play restriction or whatever. It doesn't keep checking to make sure the character is a noble right?

Staton said:

More to the point, as a dupe the Loyal Guard is considered to be a textless, titleless, traitless card. So what text would would there be to keep on checking for a Noble crest?

Remember, dupes are not considered to be attachments, so the restrictions on attachments do not apply.