We all know that the IT-section of the GCR says..
At the end of a vehicle’s fourth year of Improved Touring classification, an assessment of class equity shall be made and the vehicle’s minimum weight shall be established.
Which would seem to say that at the end of the 4th year, the weight of the car is set in stone... but that's not true.
First - Sporting regulation 1.2.2.A clearly says...
The SCCA may revise, in its sole discretion, the GCR or issue Supplements to it, at any time through “FasTrack,” “Racing Bulletins,” or “Tech Bulletins”, or on the official SCCA website. All supplements will have a published stated date.
That's pretty damn clear - SCCA can revise ANYTHING in the GCR and such revisions, by definition, cannot be in violation of ANY section of the GCR.
Second - The GCR is not the ultimate source of authority in SCCA. In the U.S., the power/authority of the legislative branch (The CRB/BOD) are constrained by what Constitution (the GCR) grants that body. In SCCA, the power/authority of the GCR (Constitution) is constrained by what the BOD[CRB] grants the GCR.
The GCR doesn't even define the rules underwhich SCCA holds club racing events. Sport regulations 3.5.1 trumps the GCR (!)
Before a sanction can be issued, the Club Racing Board must approve all Regulations different than those of the GCR.
If the Supplemental Regulations say X and the GCR says not X... the event allows X because the Supplemental Regulations are CRB-approved, one-event only alterations to the GCR.