If you had posted in the correct forum (there's a separate forum for American Express Membership Rewards cards), then there you would have seen the news regarding churning changes at Amex (the bank):

http://www.flyertalk.com/forum/ameri...etime-usa.html

So, no, in general you can't churn. But who knows, maybe there's a limit to what they call "lifetime", or maybe invitations are the exceptions? If you do apply, you should report your results (and mention it was an invitation and the exact timing) in that thread.