Or re-run the interview for your bike. You don't automatically get new classes, if there are new classes or updates to rules for existing classes you need to re-run the interview to pick up the changes.
No Ant, there are 2 classes. 250 Prod and 300 Prod, so the 300's could run and the 250's would not be hampered by the displacement. But both bikes can run in 500 Twins and 250 Superbike.
The CBR250 is now the CBR300, Yamaha trademarked the R3 and YZF-R3 names a couple months ago, and KTM might be bringing the RC390 (373cc single) to the US. The 300 class is the future.