upgrading to AP

[Kt75]

I was thinking of getting 7 day PH passes from a 3rd party (reputable) site and adding to my MDE. Was thinking to upgrade it to AP before getting to park. Is that possible or do I have to visit guest services on my 1st day use?

Also, if 3rd party pricing is less than WDW rates, I only have to pay the difference to AP rate?

Rationale to update prior to 1st day is to see AP pricing, get memory maker...