I have a lot of old AD&D 2nd Edition books, but I only have the core books for AD&D 1st Edition, plus the Deities and Demigods book. I am looking for a way to convert second edition to first, because there is way more content merging the two. Any suggestions?
Simple answer: have fun.
The class progression, stat blocks, and wordings are slightly different, but in practice 1E and 2E are pretty much interchangeable. It's comparable to the difference between 3.0 and 3.5. In particular, using an adventure from either one under the rules of the other is almost seamless. Rules-lawyering players might complain about a few details, but that's a separate issue.