[scorpio_rocks]

Sometimes you cant go directly from one class type (list in GG "MBT Unit Classes") to another but can get there by swapping to a third class first (ie perhaps APC -> tank -> SP atgm). However this will lead to two loses in experience, as the crews grapple with new equipment, and as Suhiir points out may lead to the crews having the "wrong skillset".

Often its easier and more economical to delete the core unit and expand the core with the desired replacement.