No Dayton, but it looks like they've signed up for the Tampa and Orlando regionals.
Essentially, as you move up from A to Open to World, the skills become more advanced. Originally, A class was the beginner class in WGI, but now it's regarded as more of an intermediate class. That's why in the Texas Color Guard Circuit and other local circuits nationwide you've got a number of other types of A classes besides National A (like Regional A, AA, and regular ol' A) to accommodate units with more beginner-like skills. Actually, WGI has Regional A class at their regional competitions, just not at the World Championships. So then, the three WGI classes at Dayton are A - intermediate, Open - intermediate to advanced, and World - advanced to standard setting. The classes have quite similar scoring sheets and have the same judging captions (equipment, movement, design analysis, and general effect); it's just that the average model (paradigm) that you're compared against becomes more advanced as you climb up the classes, which means that a score in one class is not comparable to a score in a different class. A class scoring is interesting in particular, because WGI weights Excellence (execution) a bit more than Vocabulary (variety/range/depth of skills demonstrated). This is to encourage a solid technical foundation, and is not done in the higher classes.
Guards can choose whatever class they want to compete in. They usually choose the one that they think best matches their skills and where they'll achieve some degree of success. It should be noted that if a guard is too advanced for their class, they're at risk of being reclassified by the WGI Steering Committee, which reviews videos of the top 2 guards in the A and Open classes at each WGI regional. (In Regional A, it's even possible for the chief judge at the regional to promote you to A class between prelims and finals!) It's not a perfect system, as we saw a couple years ago with Jordan HS, who was reclassified from A to Open and then didn't have "marked success" in Open, which typically means making semifinals at Dayton. (This year Jordan has returned to A, which will be interesting.) Technically, reclassification is different from a promotion, which is what happens at the very end of the WGI season, when the top 3 to 5 guards at the World Championships in the A and Open classes get automatically bumped up into a higher class for the next season.