Converting HeroQuest to Descent 1E might not be too difficult. It would still be a fair amount of work, but the revealing of areas would work out, and the idea of the heroes fighting their way to a boss they need to kill would remain consistent.
Converting HQ to Descent 2E would essentially amount to completely re-writing HQ. The maps in D2E are all revealed from the start, whereas HQ has the maps revealed one room at a time. In D2E the OL is able to move all of his monsters freely, whereas HQ Zargon can only move those monsters the heroes know about (unless you want to invent some house rules for double-blind play.)
The heroes in D2E don't die, they just fall down and get back up, so barring a house rule to fix that, you'd have to write in objectives for Zargon to complete in each quest, or there wouldn't be any point.
Also, from a balance perspective you may need to tweak the number of monsters running around. In HQ the combat odds were stacked in the heroes' favour (two white shields to one black shield), but in D2E the odds are much closer to 50/50. All heroes and monsters use the same attack and defense dice, nothing is loaded one way or the other.
So, you'd have to rethink the maps, the monsters, the treasure system, and probably the fluff text.