Hi Ivan and Greggory,
thank you for your input. To answer the question of how I did the migration, all I did was uplift the hdd and transplant it. I wasn't convinced it would even boot in the new machine, but after some careful experimentation (blind flailing around, that is) I got to VGA mode and then reinstalled the Matrox G400. I was lucky enough to have a Matrox installation CD lying around. Unfortunately my CD ROM drive does not show up in Warp, so I copied the relevant OS/2 driver files from the CD to a floppy disc, copied them into a folder on the Warp PC, and installed the G400 drivers from a folder. I now have 1024x768 and true colour on my Warp 3 PC. So my call for help was more of a sudden panic!
However.... my Windows 3.11fw subsystem has now developed comically large title bars and control buttons! With my Trident 9440, which was set at 1024x768 with 256 colours, it looked perfect, so I'm not sure how this has come about. Any clues on this? Should I reinstall Windows?