Normal policy for non-cash tickets (comps, awards, etc) precludes upgrading the ticket. In your case, you are not upgrading to a higher ticket. You are looking to exchange it for a Guest 10+ ticket of the same length.
Guest Relations should be able to handle this for you but I can't give you a concrete answer as to how they will handle it. It will depend largely on the CM you get when you do it. I see four possible answers they could offer you.
1) Charge you the cost difference between a Guest 3-9 4 day PH and a Guest 10+ 4 day PH.
2) Take it and convert it to a Guest 10+ ticket for no charge.
3) Tell you to go ahead and use the ticket as is
4) Tell you it is a non-cash ticket and cannot be exchanged.
If I'd have to pick the most likely option, I'd go with #2. But you'd better be prepared for any of the four possibilites to be on the safe side.