See:
https://openxcom.org/forum/index.php?topic=5047.msg167120#msg167120You can't apply transformations to soldiers which are out of the base, e.g. enroute to mission, if you try to do this through "Transformations Overview" screen; shows no eligibility for those soldiers that are out of the base.
If you go to the agent directly and apply it there (see the instructions to reproduce above), you can do it and it results in strangeness (the agent teleports back to the base).
I suppose the same restrictions should apply as in "Transformations Overview" screen.