If you put a 29er suspension fork on a 26er frame it will jack up your head tube angle. In an area where people debate about single degrees, this is a HUGE deal. Good handling 29ers actually have steeper head tube angles then 26ers intended for the same purpose. You're going in the opposite direction.

You can put a suspension corrected 26" RIGID fork on the bike and this should fit a 29er wheel. It won't jack the angles up nearly as badly (remember, you'll still have an extra 1.5" between the axel and the ground). But that is STILL pretty bad as you're going the opposite direction you should.

If you want to go 69er, buy a 69er frame. Though honestly, I see little benefit to a 69er beyond allowing for more suspension clearance. The "greater inertia" often cited by with just a little knowledge of physics (but not enough) is actually the difference in gearing between a larger wheel and a smaller wheel. I can ramp my XT/Rhynolite 29er front with 2.35 tires mounted up to 7mph with just a flick up my pinkie finger. The "slow acceleration" argument is just a fundamental misunderstanding of gearing.