As CF said, you can do it all at once if going to an AP. The procedures were updated to allow for it, but there is still the possibility of the CM not known this and tell you you need to enter the park first, in which case you get the 1-day ticket, go into the park, and then go to Guest Services and complete the upgrade.
Doing it right at Will Call saves you time, that's it.
If not upgrading to an AP (say, just a regular MYW ticket with more days), you are supposed to go into the park first to activate the ticket - but some CMs have also done the upgrades immediately.