We first used our ticket AND upgraded them to an AP on the same day. Is that not common practice when you purchase from a 3rd party and want to bridge to keep the discount? Walked through the turnstile and right back out to the GS ticket counter, upgraded for exact cost I expected.
For what it's worth,
UT will inform you the price you are saving over retail when you purchase. It's part of their chart. I found the 6 or so (I forget exact number now) day PH WP&M ticket to be the biggest discount. About $35 or so, per ticket. Since we used the ticket the first time at a park and not a WP we were able to upgrade to the standard pass.
The only drawback to this method is you can only book FP for the number of days on the ticket you buy. In our case we have old Ghost tickets on
MDE that work for FP booking even though they don't have admission left on them. So it didn't hurt us to wait to do our upgrade. If we didn't have those the savings is too small to make it worth missing the FP