I'm looking at a 1999 Trek 5500 frame with an ahead stem. Is it possible to change this to a quill stem? if not, why not?

I have been told that in some weird way, these older versions of this frame could be installed with either ahead or quill. But I asked someone who had his for sale, mounted with an ahead stem, whether it could be exchanged for a quill, and he said "yes, but you need a new fork?" - no further explanation. How does the fork matter in this?

Thanks in advance. The specs catalog from 1999 doesn't offer an explanation. It only says the fork is an ICON Air Rail, and shows a diagram of the head cups installation.