You can absolutely use the 28.5 in the 29.5. The front of these are very thin and flexible so they will stretch to fit and utilize that extra shell space. I use the same 27.5 Zips in both my 27.5 and 28.5 boots and honestly, the Zips themselves feel like they were perfectly made for either size shell.
As I have looked into I probably need the Freeride model give my wide feet and high volume boot. Also think I need to try one on first to make sure it fits and have a boot fitter properly fit it. So with that in mind I think spending the extra $50 to buy it new is the way I am going to goWhat he ^ said ^ @dovski