According the new bravo special series, Reality for Reality (not sure if I have the correct name, sorry!). Contestants have EVERY right to sue should the producers interfere with the outcome of the show. I'm not sure what the contract for PH guest states, but accoring to producers interviewed last night for this new series -- these contestants have EVERY right to sue the producers unless it stated that the producers have the right to change their minds and make rules up as they come. Producers are not allowed to make guests vote the way they want them to (Stillman suit), nor are they allowed to interfere with who gets voted off and when due to changes in the rules.
In this case, if I were Dave/Charla/Keith/Tara, if I did not come out a winner due to this new "twist" allowing old guests back on to visit and then stay, I would sue the PANTS off of Fox! So Barbie 4 if you are reading this, SUE Fox for all they are worth!
Then again, the gag rule just ended for season one of Survivor and Joel Klug stated that there is a lot to be said about what really happened but he has seen what has happened to those who have made a beef about things and so for now he is NOT going to speak his mind.