IMO, converting from 1e/2e to 3e is extremely easy (so much so a monkey could do it!). It's especially good because it's so easy to determine what the "new" appropriate levels will be for 1e/2e adventures because you can determine an EL for every encounter (which is what I do in all my conversions). GDQ1-7 is what I use as a "baseline" for conversion difficulty; IMO, it's probably the most difficult older adventure to convert due to the fact that the ELs are very high in the G series, then drop lower in the D series, then get all loopy in the Q series. Therefore, a good amount of work is needed in the D series (however, I've discovered that GDQ is the most rewarding to convert, especially with some of the info in a lot of the d20 books that can be added, such as BoVD, R&R, R&R2, BoEM, BoEM2, and even the splatbooks). And, really, it isn't *that* hard. You're just advancing monsters, or simply adding some class levels. Easy.
Going from 3e back to previous editions would likely be very difficult. I was considering the consequences of doing so back when 3e was introduced (because my group actually *liked* 2e). I discovered that it would just be too difficult to retrofit 3e rules back to 2e. For me, anyways.
Anything to make the DM's life (mine) easier!