Disney does not penalize a person for buying discounted tickets when upgrading them.
If the ticket has been purchased at a price less than the current gate price this is a two-step process. Note it can only be done within 14 days of first use, even if it has the no-expire option.
First an "intermediate" ticket is issued, which brings the price value up to the current Disney gate price. This is charged to an internal account and not to the Guest.
Then the intermediate ticket is upgraded to a new ticket by adding days and/or options. All the Guest has to pay is the cost for each day and/or option added (plus tax, of course).
Note this only works for MYW tickets. Anything pre-MYW cannot be upgraded; if completely unused you can apply the cost value to the new ticket. If partially used nothing can be done but you can still use the remaining entries. If demagnetized we can replace it.
(I work at a ticket/Guest Services location.)