For those [lucky] few who have the book, I'm not sure if you've pondered this at all but how hard do you think it would be to run the older adventures or sourcebooks with the current ruleset?

From what little I know it seems that netrunning/hacking/decking has drastically changed, but might not be so hard to "port" backwards 20 years. Wireless networks are not all that new anyway...

Magic mechanics seem to have definitely changed, but has the concept done so as well? From what I've seen it doesn't appear so, so that might be easy to take backwards in time.

I do ask this because I have been so eager to run Harlequin and other early adventures (NAN1, 2, etc.) for my group as I see them as awesome introductions into the SR world. Well, that and I'm not in love with the whole 'Otaku/Technomancer' concept, but that's a personal thing

I actually updated the downloadable SR3 Missions runs for use over the weekend and it was pretty simple. I used goons from the SR4 book along with their lieutenants and things went pretty well actually. Had to come up with some firewall ratings on the fly as people began to hack things that weren't hackable in third edition (at least not easily hackable) but things went very smoothly. Not sure about going as far back as SR2, but SR3 converts with a moderate amount of effort focused on the NPCs and the matrix stuff.

Yeah, that works if you take a 2060 scenario and bring in into 2070. But if you want to play a 2060 scenario without Augmented Reality and the new Matrix, are you pretty much screwed?

Basically, what I'm asking is, is there a way to play SR3 Deckers using SR4 rules?

SR4 includes both AR (Augmented Reality) and VR (old school Virtual Reality). Strip the AR out and just use the VR and you have the setting deckers used to play in, essentially.