Ohh, I have to get back to work, but this darn twist has me hooked right now, so...First: Stacey's lawsuit is in serious trouble, and it was in serious trouble during the filming of S3. The problem that she has is that the ONLY evidence for her claim is Dirk Been's statement -- and Dirk's statement contradicted an earlier Dirk letter. At some point, whoever is trying this case for her has to fold and go home.
Second: I would expect that Castaway Productions uses a similar agreement worldwide regarding the right to realign teams -- in other words, I expect that the language in the Swedish agreement is virtually identical to the language in the US agreement. Therefore, I doubt that anything prohibits MB from making such a swap.
Third: Keep in mind that anybody can sue anybody else about anything. Therefore, a member of the GXA could sue MB. However, the suit wouldn't survive summary judgment IF MB could show that he was planning to make the change before the game started. Frankly, I presume that he was planning to do just that; he probably even asked some of the people from S1 and S2 about it ahead of time (I'll bet that the Tagi 4 were all asked, which might be why Soozin has legit spoiler info in her TV Guide article).
What would CBS lawyers advise him? Well, if he's doing a pre-planned change, I'll bet that they've already examined the agreements to be sure that he would defeat any lawsuit in a summary judgment motion. He would only have a problem if this change was a gimmick developed in response to the GXA.
In non-legal terms, the standard is to "play fair" (which is different from playing nice, BTW). If this change would have happened regardless of the game situation, then it's consistent with playing fair, and CBS's lawyers would approve of it.
No sig pics, please, I'm allergic.