There is no command "resistance side". There is a command "side", which returns the side of a unit. For example, you could have this line in a script:-
? side _loon1 == resistance: goto "next"
Syntax not guaranteed, that's just a demo. "Resistance" does appear in the comref as one of the four possible things that the side command can return. The others are of course west, east and civilian. Hope that clears things up. When you're not sure about a command, always check both the online version of the command reference and also the Unofficial comref - between them they almost always explain how to use it.
There is no way to change the side of a unit in a mission, in fact you can't even change it in the editor. Depending on circumstances, it is possible to fake it but from what you've said it sounds like Kali's solution is what you need.