Downgrading from running RoS version to any earlier one (down to the factory installed one, no earlier) is always possible and relatively easy, see:
http://forum.mikrotik.com/t/downgrading-routeros/165214/1
There has been a change in wireless packaging/modules between 7.12.x and 7.13, but that shouldn’t be a problem, the 7.12.x contains all, while in 7.13.x and up you may need the separate package:
http://forum.mikrotik.com/t/v7-13-5-stable-is-released/171923/1
the upgrade process from 7.12 should take care of that automatically (if you use the built in procedure).