I would not trust the resort concierge locations to correctly upgrade to AP.
In the past (and may very well still be this way,) the resorts' computer system did not have the same
operations and accesses as Guest Relations and ticket booths.
However, it may be possible that once you are actually at WDW, your MDX account will give YOU the option of upgrading to AP.
This option MIGHT not appear on MDX until after you have entered your first WDW park gate.
But, that's not a problem or detriment in regard to the cost of upgrading.
If you have your
UT in your MDX, have you (even right now) checked your MDX for the option of upgrading to AP?