Is B/X not the same as BECMI (OK, without the later sets)?
My understanding is that there are quite a lot of differences, although none of them important enough to prevent people from mixing and matching modules. One of the main changes was in the tone, I believe, due to different authors and possibly a bit more political correctness creeping into the later versions with a general steering away from religion (e.g. as pertaining to clerics). I can't be sure because I've never really used the BECMI rules despite owning the Rules Cyclopedia. I can't say I'm wedded to the B/X rules in any way except for nostalgia reasons, and when I do the personalised version of my B/X book I'll include some things which I actually liked from 3.5, like d20 skills and ascending AC (and divorcing race from class).
Bleh, percentile strength, that was the first bit in AD&D1 that put me off (there were many other bits later).