The differences between AD&D 1st edition and AD&D 2nd edition is very small 2nd edition is merely a polished version of 1st edition so you shouldn't really have a problem using 1st edition modules in 2nd edition.
I asked a similar question here the answer by aramis offers a comprehensive detailing of the differences between the editions which might be of some assistance.
Also I took a look at the differences in the stat blocks from AD&D 1st and AD&D 2nd editions and they are basically identical, the 2nd edition stat blocks are just a little more organized.
I haven't played 2nd edition but I have used 2nd edition campaign settings and adventure modules in my AD&D 1st edition campaigns and I had absolutely no trouble using them, there wasn't anything i found incompatible. I basically did what your trying to do but in reverse and it worked out fine.