Alleytom

Now that I have a road bike I've been setting my hybrid up more for longer distance riding, with some light trail riding thrown in, and adding water (for desert riding) & storage. So, if I swapped my 700x40 tires for 700x52 tires (AKA 29x2.1") would my old GT Arette become a 29er bike?

It looks like I have the clearance for the larger tires...

hokiefyd 

Yes -- all of these terms are colloquial/marketing in any case. Your wheel rims have a 622mm bead seat diameter (BSD), and you'll almost always see this in the ISO size imprinted somewhere on the tire, usually in small characters. Your 700x40 tires might actually say 40-622 or 38-622 on them. A 29x2.1 might actually say 52-622 or 50-622 on it somewhere. They all fit 622mm BSD rims and a "29er" tire is nothing more than a large volume tire that goes on the same rims as "700c" tires.

It looks like your rear brake caliper may present the tightest clearance (or possibly your chain stays). What tires are those? The tread pattern looks like Michelin City/Proteks to me, and those sometimes run larger than stated. So they may actually be more like 42 or 44mm wide/tall. And a 52-622 may actually be more like 48mm wide/tall, depending on the specific one you buy, so your actual mounted width and height may not change as much as you think it might.

Alleytom

You were right about the chain stays, 50mm max where the tire is so no 29er for me, and the tires on it are City/Protecs and are right at 40mm wide. My plan is to have a set of wheels/tires more trail oriented, I'll have stick with 700x45 max I guess. Have a cheap but decent spare rear wheel, need another accushift freewheel though or I'll have to swap the freewheel when swapping wheels. Need a front wheel and I'll be ready to roll.