Upgrading tickets is done at Guest Services at the resorts or Guest Relations near the turnstiles both inside and outside parks. Package tickets, once handed to you, may be upgraded one at a time.
A ticket in hand (purchased perhaps in advance at a discount) cannot be combined with another ticket in hand (perhaps part of a package).
One (standard; not promotional or convention) ticket may be applied toward purchase of another ticket with the difference paid in cash -- gate price versus gate price. If the ticket in hand is partly used then there are restrictions as to what you can trade up to.
You may purchase tickets at a discount for use during your package vacation while hanging on to your package tickets unused for the future.
Using both the package tickets and tickets purchased at a discount elsewhere during the same 4 to 10 day vacation is not recommended.
If you are having a hard time deciding whether to buy or add non-expiration, use thgis as a tie breaker: You should foresee some future vacation where the remains of the ticket, perhaps with some Universal Studios or Sea World or Busch Gardens or Ripley's Believe it or Not or Medieval Gardens fun added, is all you need.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm