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.
1E and 2E are pretty much interchangeable. Compared to the upheavals in 3.X and beyond, 2E is effectively 1.5E. Rules-lawyering players might complain about a few details, but that's a separate issue.