AD&D adventures can be converted pretty easily. My current campaign is often called the Greyhawk Classics campaign, because I'm running the best (more or less) of the Greyhawk Adventures by EGG. Temple of Elemental Evil required only a moderate amount of work (I had to change some creatures in the Elemental Nodes). Forgotten Temple of Tharizdun and the Hidden Caverns of Tsojcanth needed more monster replacements until I got Fifth Edition Foes. Against the Giants had scale and balance issues, so it was even harder. Oddly, Descent into the Depths of the Earth has been the easiest to translate so far.
If you do this, you'll need to cut down the treasure by at least 90%, since the amount of treasure assumed paying for training. Depending on your group and playstyle, you might want to retool the encounters a bit. When using giants or other large creatures, make sure to check the size of the encounter works (sizes have changed quite a bit since then, and you might need to change the scale a bit).
