I believe the station can not just decide that at any given time. I seem to remember there is a time frame for the station to decide. I don't know that they can change, or at least change quickly their status.
EDIT - I found it- they can only change every three years, next time is 2014 I believe.
But if a broadcaster had chosen 'retrans', and decide they MUST be on (change their mind to 'must carry'), they could always ask for some extremely low amount (1 penny/month/subscriber).
If a broadcaster had chosen 'must carry', they're stuck with it until 2015.
BTW, I personally don't buy into the "they want to charge us 200% more" claim. Sure sounds like a lot, but what if the old agreement was only .05/month/subscriber? 200% more would be a total of .15/month/subscriber for the new charge. Are you willing to pay a dime more a month for your local? Granted, it could be MUCH more. But since Dish is making that claim, they're trying to spin it so they sound good. How do you think it would sound if Dish said "they want to charge us a quarter more for each subscriber"? Doesn't really sound like that big a deal, does it?