Can I swap a 29er fork inplace of a 27.5 if I maintain close to the same axle to crown length?
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.