Topic says it all. How should I treat the change stance action, i.e. a PC is changing his/her position form standing to crouching or going prone. Rulebook doesn't say anything about this kind of thing (not surprising really). Should I treat it as a half or a free action? Making it a full one is a bit tough, in my opinion. What do you guys think about this?

I tend not to have my games get as specific as "crouching" or "standing." I treat prone as a half action to get down into/out of, but go with the book in that there is no necessary benefit to being prone except maybe a modifier to stealth checks (or rather a penalty to the opposed perception check).

Crouching seems a rather simple, natural, and fluid aspect of normal movement. And I leave it at providing no further benefit over normal movement. For the purpose of crouching behind cover I just ask that players indicate when/where a PC is attempting to use cover, and make an on the fly ruling about the amount of cover it can actually provide.

For Prone to Standing, I say look at "Leap Up" Talent which lets you get up as a Free Action, in the Combat/Moving section, it is a half action to stand up. To be simple, use those and let players crouch/stand upright with ease, as long as they were in one of those positions before.