I was reading the Star Trek Encyclopedia the other day and I noted the description of the warp scale change from TOS to TNG. Apparently Gene Roddenberry stipulated that warp 10 was to be the absolute maximum for warp drive in TNG. Has any timeframe ever been established for this change? It would seem reasonable that ships in the TOS movie era still travelled at TOS warp scale speeds. Therefore, I would assume that the change occurred after STVI but prior to TNGS1. I can't recall this ever being properly mentioned in any canon or non-canon Star Trek material. Just curious if anyone else knows?