Yes, sorry! I didn't mean you couldn't upgrade on an AP blockout day.
But, your ticket cannot be completely used, or past the 14 day expiration period (if your ticket expires in 14 days). If you can use your ticket to get into the park, you can upgrade it that day. And, you can upgrade it once you get into the park, you don't do it at the ticket booth. Most 5 day parkhoppers don't have any blockout days, so that's why I said your ticket (meaning your parkhopper) must be valid.
Also, if you're parking at a paid Disney parking lot, bring your parking receipt if you are getting a premium AP or if you want parking added onto your other AP. They will credit the price of parking for that day towards the parking portion of your pass.