Atlantis: Can you upgrade while you are there?

[mevelandry]

Hi!

According to many calendars I have found, our next trip is happening at the end of US spring break. I read that there was horrible crowds in Aquaventure Atlantis during Spring Break.

Is it possible to book just the Beach Day (+ exhibitions) and then, if the crowds are not too heavy, upgrade to Aquaventure on site?

Thanks in advance.