karajorma, that's a pretty cool idea about switching ship models.
If this were to go through, there would be only four types of playable ship models: the Pyro-GX from D1/2, and the Phoenix, Magnum, and Pyro-GL from D3. (The Black Pyro in D3 is basically a version of the Pyro-GX.) Add to that any robot models that would be included, and the rest would be left for multiple ship models.
You'd also need models for each level though. Still worst comes to worst you could use an inferno build and lose multiplayer if need be.
One think I unfortunately just thought of regarding weapons changes would be the number of combinations of primary and secondary weapons (a.k.a. guns and missiles). If you wanted to change from one primary/secondary to another, while still maintaining the other category's availability, wouldn't that set up a whole mess of combinations? (i.e., if you had lasers selected and wanted to switch to Vulcan, but wanted to keep your Concussion missiles, would you need separate models for both lasers and Vulcan with Concussions? If you then wanted to switch to Homing missiles, would you need two more models?) Or am I just reading too much into things? (I certainly hope so, since if I'm not, we'd probably just be stuck with the regular FS method of carrying only a few weapons banks at a time.)
If someone is interested in converting something, there's a 1500-poly version of the Pyro-GX available in D3's model format on
PlanetDescent (I believe the version on the site is packaged in a .hog file). The model actually looks pretty damn good for D3 (1500 polies for a model is a serious amount for the Fusion engine as it now stands). If someone wants to take it on, I'd be more than happy to extract the necessary files from the .hog.
This is the point when I wish I had some clue how to go about a project like this, because I'd love to see this mod happen. Realistically, though, I have absolutely no skill/experience/knowledge about modeling/converting, table files, FREDding, or the whole shebang. Still, I love Descent so much that there's a little voice inside of me saying, "Come on! You can do it!" That little voice is heavily outweighed by the more realistic, "You're the last person on the planet who'd be able to do this."

Still, if anyone ever does do something like this, I'd try to lend a hand in any way possible, even if it was nothing more than a consulting role or beta-tester.