I recently (a few minutes ago) posted a Boy Meets World meme on my Facebook page, which then led to my commenting on the retcon that happened midway through the series, wherein Topanga had changed from Corey’s hippie sometimes friend, sometimes crush, sometimes just indifferent classmate, to his own true love, which was now how things had always been, and the hippie, weird girl aspect of her character was no longer even referenced.
Now, the real reason is of course, the writers changed the the direction of the show, and needed that extra sweet romance hook, which totally worked for me, but forreal, what happened there? Like in the “reality” of the show.
I propose that at some point in between seasons, Eric Matthews, Corey’s older, popular brother, somehow had an incident with a time machine, perhaps one of Steve Urkel’s invention, as all the TGIF shows were in a shared universe not unlike that of the NBC comedies of the 90s (Remember that Blackout storyline that went across all the Must See TV comedies at one point?).
So, at some point, he goes back in time, accidentally changes a few things, and then gets himself a head injury somehow, perhaps in fixing another timeline error, as when he comes back, things are the same, but different. Corey and Topanga had been together since elementary school, and she was never a hippie, their sister now looked like a different person, and Eric was the loveable but dumb brother. And to them, this is how things always happened, but only we know, and maybe Eric, somewhere in his mind, remember the other timeline. This show totally did the Abrams style reboot 20 years before that was a thing, or before we knew JJ Abrams could do things that weren’t Felicity.