IN THEORY, could you switch the components from a Hardtail 26" MTB to a Hardtail 29" MTB?

I know the following would need to be replaced:
- Frame
- Front Fork
- Wheels/Tires

Thanks!

Sure. Brake lines may need to be lengthened too.

Would suggest, unless bought bikes have the same specs, you will need a lot of additional components. take any 2 MTB's and and you could have no interchangeable parts. Would be creating a list and get the specs for the 26" && 29 and find what matches before continuing.

Awesome. I MAY be getting a 29" frame for free that is significantly lighter than my 26" frame and every says 29" is the way to go! The components on my bike are solid so for about $300 (I would get a cheaper fork) I could have a solid 29" MTB.

Well bottom brackets/crank maybe and issue too but that is more frame differences than 26 vs 29er. And front derailleur and seatpost may also have to be changed out but those are cheap or at least can be cheap.

You might need a new seatpost too. Seat tubes come in different sizes.

Greetings
Sell the 29er frame, keep, and upgrade your 26er bike. 26in. wheels are lighter, and stronger than 29er (700c) wheels, your 26er fork is lighter as well. 29er bikes are a marketing move to sell new bikes, and perhaps the American love of bigger is better! I tried a 29er, and honestly, it was heavy, not responsive, and didn't handle nearly as well as my Tange Prestige framed 26er. Just my 2 cents.
Regards

Wheel strength is a legitimate concern I have. I currently weigh in the 280s and need strong wheels...

Greetings again, I've been riding and wrenching for 40+ years, and wheel physics 101: the bigger (diameter) the wheel, the weaker the wheel. The new 27.5 (6650b) size makes a bit more sense than 29ers. I weigh 225lbs. and riding mtb's since 85, bikes are not rocket science, so stick with 26in. wheels, they are lighter, stronger, more responsive,and handle better! Good riding.
Regards