Can I swap a 29er fork inplace of a 27.5 if I maintain close to the same axle to crown length? I am looking for some verification of thought process. I have come across a nearly new lower level Diamonmdback 27.5" hardtail MTB. The issue is the front 100mm fork is broken. Doing some online searching for a replacement fork has OEM budget forks (SuntourXCT) for roughly half of the bikes original cost. I can however purchase a 29er fork(Rockshox xc32) with some deep discounts for less than the Suntour. I realize the longer dropout to crown length of the 29er (19 1/4" vs 17 1/4" for a 27.5) will alter the handling. If I got an 80mm 29er fork, that difference would be cut in half. My thinking is that the 1" difference in length would be similar to the difference of the fork length when set to "soft" dampening  versus "firm" dampening while static.
As the axle is also higher, you need to really consider the Ground to Crown height rather than the axle to ground height. Also as the 29er is a larger diameter wheel, it may not clear the down tube, and may interfere with the feet when the pedals are horizontal.   

Combining all this and the fact the geometry will alter the handling somewhat in ways that may or may not be significant depending on the bike and your person riding style it seems to be a fairly big risk. Only you can decide if its worth it.