Careful with your terminology.
It IS too late to upgrade them.
However, you can receive a pro-rated dollar credit towards purchase of a new admission.
When you or I think of upgrading, we think of getting admission that has more days or more options than what we currently have.
When a Disney cast member speaks of upgrading, they're talking about getting back the ENTIRE value of what you paid for the ticket towards more days or more options.
Seems trivial and nit-picky, but if you ask if the ticket can be upgraded, some lazy cast members may tell you no, and not let you know that you can get the dollar credit.