Perhaps a silly question, but I am new to mtb-ing. I apologize if this has been covered. I did a search and couldn't find any information.

So here it is... Can a 29er be converted to a 26er by simply replacing the wheels (and doing whatever needs to be done to brakes)? Are there 29er-specific forks? Are the seat stays and chain stays on a 26er too short to take a 29" wheel? Does it vary depending on manufacturer? Specifically, could an Orbea Lanza 29er be converted to a 26er?

I know there are 29er specific forks.
I'd imagin the geometry would be different also.

Yes you can, but the geometry would probably be realy weird.

It'd be like taking a big lifted up F-350 with 34's on it and putting 18's on.
lol.

Bottom bracket height would be lowered...

And yes, chainstays/seatstays on a 26er are too short for 29er wheels. Unless you use hybrid/cross sized tires...

...why are you wanting to convert? Most people are doing the opposite...

I am wanting to purchase a mtb and as I looked at the 26 vs. 29 debate (which rages large on some sites) it made me wonder if the wheels were interchangable. Couldn't find any information so I thought I'd ask the experts. Thanks.

As long as you have disc brakes, the swap should be feasible. I don't think much of the geometry would be affected adversely, but the lower bottom bracket could be a concern. YMMV.