Can you use jokers to change which noble visits at the end of turn?
No, you cannot.

From the rules:


  At the end of their turn, a player automatically receives the visit from a noble if that player has the amount of bonuses (and only bonuses) required.


(emphasis mine)

A visit from a noble is determined only by the bonuses printed on the development cards.  Tokens, including the Joker token, cannot be used to get a visit from a noble.