I'm sure you have read the
specs, as you seem to know a lot already about this. The thing is, the hex values for the tiles are so different, a 1-to-1 conversion is very hard to achieve, and often ruins your ISOM section as well. Your best bet would probably be to jack up your desktop resolution, fire up SCMDraft, and with the source map and a blank map open simultaneously, copy the terrain freehandedly.
That is, of course, unless SI or Heimdal comes in blazing with a radical new tool they designed especially for this purpose, but have never publically released before

UED77