Buster Sword does kinda sorta retcon the after-credits cinematic of CC, unless the modification comes a little later, which I find unlikely.
In fact, I would love so much if it was noted in-game that Cloud had ended up at the trainstation with full SOLDIER uniform and modified it later on, before his first mission as a mercenary. Buster Sword could then be modified later on (if that's ever to happen and it's not like that from the beginning), perhaps after running into someone who recognized the weapon during his first mission?