There is a rule that if you are racing East you may not race West and vice-versa. The exception is, or has been, that if you sign-up for West but are injured before the series starts or at the first round (without scoring any points) you can switch to the East coast instead.
Also, most all of the top ten or more in the 450's hve pointed out of the 250 class, thus keeping them from being eligable to drop down. The 250 riders signing up for the 450's is the only way one class crosses into the other and I think it has worked well over the years. This has been happening since the birth of a East/West coast feeder class and I don't remember there ever having been an issue with a Lites rider screwing up the show for one of the big boys.
And isn't there a rule now that you can't just sign-up for a SX race but first have to go prove yourself by scoring a certain number of points in the Arenacross series? It seems like I heard that somewhere.
07 Yamaha R6
07 KTM 950 Super Enduro
04 KTM 250exc- plated two-stroke
01 Kawasaki KE100
74 Honda MR50