You're talking about a couple different things now.
First of all, the Sprint M8 already supports some (not all) LTE bands, out of the box, with no modification. Maybe the bands supported (there are many used by various carriers worldwide) are not the ones required by your local carrier. Or even if it supports the proper bands, that is no guarantee you will have LTE service on your local carrier; due to carrier specific deployments of LTE. From what I've seen, LTE support on various carriers is very tricky and there is no consistency (even when the device supports the proper LTE bands).
I'm fairly certain that some folks on here have had success getting LTE to work on their local networks on the CDMA M8 (at least Verizon). But your results will vary depending on your local carrier and the factors just mentioned. Plus, the Sprint version seems more locked down than the Verizon M8, so yet more complications there.
Second, what you did trying to convert the Sprint M8 (which you did not previously mention, and a completely separate matter) by flashing RUU, firmware, or radio from any other M8 version is definitely impossible, as you mentioned due to hardware differences; in addition to different radios, different partitioning, and different kernels. This is simply the wrong way to try to change band support on a CDMA version M8; which will not work and may (as you know) possibly lead to a radio brick (and sometimes a permanent radio brick).
There are some possible mods to modify band support on the CDMA variants. However, your results again may vary. In particular, it seems the Sprint version is not particularly compatible with such mods. But here they are, in any case. As always, proceed at your own risk, if you so decide to. And only after reading through the threads and understanding properly :
thanks sir for reply