2e revised was compatible with 2e material, and 2e was also largely compatible with 1e. It wasn't til 3e that we got really DIFFERENT editions that would require serious conversions. 3e material was still largely usable with 3.5.
They made their revised material compatible with the previous for the reasons that 5.5/5e2024/whatever is "not a new edition." I couldn't tell you what's different this time around, though, why WotC's not calling it a new edition... I guess it's their "we're happy with 5e, OneD&D is an evolution but not a new edition" stance. Whatever it's called, we assume that we know what it is based on previous edition iterations: 5.5