The most significant thing about this upgrade is that it actually makes it possible to play the game through to completion without resorting to the cheat codes. The initial release had a bug which would cause the game to hang (or crash) at certain times, for instance near the start of the final level of the Dome of D'Sparil episode (ie. the climax of Heretic).
Previously the solution to the problem required the use of the cheat code which allows you to walk through walls, etc. (and therefore avoided triggering the events which caused the problem) - hardly ideal. I'm pleased to say that this is now a thing of the past and I finally got the chance to make toast of D'Sparil properly.
The upgrade isn't just a bugfix though... The texture resampling has been enhanced to cope with floors and ceilings as well as walls and 'things'. This improves the smoothness of the display significantly, especially where constructions like stairs and ledges are involved. Previously the resampling would only affect the vertical components of such things giving a somewhat strange effect: smooth vertical surfaces and blocky, pixelated horizontal ones. Now all the surfaces are smoothed things look much better.
I did have trouble with the resampling when using the 'fixed resolution' version of the game, however. The variable resolution versions were fine though so the problem can't be that significant and hopefully will be fixed very soon. This probably isn't much of an issue anyway... if you're using the fixed resolution versions then it's probably because you're using a slightly slower machine (i.e. non-StrongARM) which isn't really up to the heavy duty work required for the resampling anyway.
Overall, not a particularly exciting upgrade, but one that's essential to properly enjoy the game. The new resampling is very nice although, as with the original Acorn Heretic, it's only really relevant to StrongARM users.