I GOT IT TO WORK!
The creature I wanted to switch to (my companion) was listed as "1" on the list of available creatures (the character I was currently playing as was listed as "0"). Typing in "9" caused it to switch to my companion properly.
EDIT: Ehehehehe . . . this can be a little erratic. Trying to swap to various numbers (including the number of my companion, and the number of my companion+8) while on the site of a lair did *not* result in crashes, impressively, but instead transplanted my soul into random creatures inhabiting the cave systems below me.
DOUBLE EDIT: Okay, it seems the problem is that the name of the creature you are supposedly are going to swap to and the creature the program actually swaps you to are not properly correlated with one another. At the site of the lair, the list of swap targets is 1:bemta (me), 2:sapa (my companion), and 0:zakosp (the night creature we just killed). Typing in "0" or "zakosp" swaps me to myself (no visible effect; this is consistent with what I had observed earlier- 0 is "yourself"). Typing in "1" or "bemta" swaps me to the deceased night creature. Typing in "2" or "sapa" swaps me to a monitor lizard fiend in Hell. Note that these are all reproducible; I can swap back without causing a crash, and repeating the input swaps me to the same target as previously. The fact that it is noticing the night creature as a listed swap target is also a little odd, as the selected race is "human", but I suppose it is technically a transformed human? I don't know.
Typing in "3" swapped me to a draltha in the caves, however, swapping back failed to work properly. When I savescummed and tried it again, it swapped me to a monitor lizard fiend in Hell (not the same one as when swapping to 2, but it was standing right next to it). 4 and 5 swapped me to further demons in that same cluster. I think distance might factor into this somehow.
TRIPLE EDIT: Aaaaand "9" swaps me to my companion properly, despite the fact that his displayed target number in the program is different than the last time that worked. What the hell.