I’m kind of ashamed right now. I’m a 23 year old and been a fan for some 15-17 years by now and only TODAY I discovered this. I thought they were renamed back in 1997-1999, when the whole Prequel thing came to life.
There’s definitely no shame in thinking that, because while the episode number has always been there for ESB and ROTJ, they were never marketed as such until after the prequels - the first time an episode number was ever seen on a home-video release cover was the 2000 VHS re-release of the ‘97 SE, which had covers that tied in to the look of the Phantom Menace VHS release.
So, the episode numbers were there in the crawls but they were never really a part of the films’ names - they were called The Empire Strikes Back and Return of the Jedi until they were effectively renamed in the early 2000s.