Definitions provided in 14 CFR Part 1.1 state, “Major alteration means an alteration not listed in the aircraft, aircraft engine, or propeller specifications—

(1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or

(2) That is not done according to accepted practices or cannot be done by elementary operations.”

Note that Advisory Circular 43.13-1b/2b, which is by definition acceptable data, covers basic tow hook installation. Flipping the hook-- arguably a removable, already installed appliance that effectively changes nothing on the airframe itself-- isn't a major mod. Since Schweizer isn't around anymore to adjudicate, just do it if you're convinced it's safer. Or better yet, buy the Tost.

Larry Ruggiero A&P/IA, tow pilot