Initial Change with Preverbs

In Inglis (1986) she mentions that preverbs also undergo initial change (contraction) when there preceded by additional preverbs. I know we have seen this with preverbs like pema- ‘along’ becoming pma-┬áin irrealis context such as in the future tense. For those of you who have looked into preverbs, does this only happen with preverbs in initial position, or does it happen more generally to every preverb with another preverb preceding it?