Looking at the Warlock Changeling spell rules (RMC2 3.2), it does take a little bit to fully grasp how all the different possible modification options relate to and interact with each other. The physical options are pretty straightforward (e.g., extra arms, adding wings, etc.), but the numerical ones can be headscratchers (giving a +5 bonus to gill slots?). The special mods listed on the Alternative Static Action Table 11.2 do help clarify that by noting specific skill, stat, and RR bonus modifications.
However, one thing that's not explicitly mentioned anywhere is whether the modifications are cumulative with repeated applications of the same effect. In other words, is the baseline for the desired modification set at where the target already is, or is it from where it originally started? For example, if a +5 Strength bonus was successfully added once and a +10 Strength bonus was attempted later, would you end up with a cumulative +15 bonus or a replaced +10 bonus?
I want to say that everything should be based on the current state of where the target is, meaning that all changes are cumulative, but I'm curious to hear any other thoughts about it.