Read the terms of your agreement very carefully. If your agreement clearly states that the maximum interest rate is 20%, then you would be on solid legal grounds to sue them when they raised it to 27%. I suspect there is something more to this than what's been posted here, since this just seems such a clear a case of bank fraud to me.
No the problem is once they are into you, they sell the account to someone else and they send you a notice of new terms. Your choice is to either pay off the debt immediately or live with the new terms.
That's how they get the rate up to 27% and current law allows them to do it. And they don't have to sell the account to change it. All they have to do is send the notice. But often they will sell it.