We did exactly that at St. Louis back in March. We asked for a change of STU from the big bore GT/EP/T1 to the SS/SM/T2 which was split from the IT group by 3 groups instead of back to back. We emailed the chief steward early in the week. He took the action request to the stewards on the Friday night before the race weekend. As long as all of your STU entrants are in agreement about the move it should be a no brainer. The officials were VERY EAGER to please as they needed the extra entries. All handled very professionally and efficient. Just talk to your chief steward and they will do their best to make it happen.