I never really converted any DOS mod to SDL, but I can tell you some things from a theoretic perspective of mine. If you want to convert a somewhat "vanilla" game (code wise, I mean), you probably won't need to do anything or very few changes in the code. However, if you want to convert a more advanced game, it may be a bit harder to do that. As for the maps, old DOS games that have textured floors/ceilings probably won't use plane 3 in their maps, while SDL games do. In that case, I suppose you would have to "transfer" the hard coded textured floors and ceilings to a map's third plane.
In other words, it all depends on which game we're talking about. Some may cause you a little more trouble than others. Like I said, I never did the process, so that's what came in my mind right now. Perhaps Andy could share a bit of his experience with you