What Rob reports is partly the old rules. They have actually changed slightly. Yes, you can upgrade and should link them to MDE for FP. The rule about using them to enter a park, was old. It probably doesn't matter anymore, but he's right it used to be how one locked in the value.
What IS STILL important is that you KNOW (or rather have some idea about) the correct upgrade price, but you can't know it 100%. The upgrade price depends WHEN the discounter minted them, which is not necessarily when you bought them. If they minted your tickets before or after the most recent ticket rule change that impacts the rules and prices of your upgrade a bit.
More or less, the upgrade costs should be the difference between the current gate price of an AP, and the gate price that was in place when the tickets were minted. Most likely, your tickets were minted:
A.) Possibly: 2/11/2018-10/16/18, or
B.) 10/16/18-3/11/2019, or
C.) After 3/11/19
Though you can't know the exact correct price for sure, well sometimes CM's will quote a price that is WAY off, so you really should make sure you know what the upgrade should cost. You always have the option of asking the upgrade price, then walking away, and trying at a second location. The second CM may well tell you a different price. Your best bet is to know what you think the upgrade should cost- at least based on current gate prices.
ANOTHER thing that is now VERY important is to do the upgrade while the tickets are still valid, and before they have been completely used. More or less, I suggest upgrading them on the 1st or 2nd day you use them.
ALSO, if the ticket you buy includes Water parks, and you use it to enter a water park, then you would have the buy the AP which includes WP access. (If you don't use it to enter a WP, then it won't matter.)
ALSO keep in mind the 6.5% Florida taxes when calculating the price.
If a CM quotes you a price that sounds too high, and it happens, you can walk away and try again later at a different. The parks also now have booths specifically set up for ticket transactions, and those may well have shorter lines than traditional GS locations. In MK the ticket office is located just behind/under the new Muppets Liberty Square show. Few folks know this office exists. DS also has a ticket booth+ GS. In my experience though the DS GS gets long waits.
OH, also make sure you always carry ID plus the actual AP. After you get home, you an request a specific AP magic band, but you still always might ID and AP to get discounts and even to get into the park.
Last if you want to do the upgrade at a park, and end up paying parking, tell the CM. AP's include free parking (at parks) so you can typically get the parking costs. Same thing it you bought Memory maker. An AP includes Memory maker, so no need to buy it. If you already bought it, ask about getting the cost reimbursed when you buy AP.