Is there a way to offer monthly or annual signup under one subscription level?

First, a quick question – I’m reading the manual for Membership, but can’t find any reference to the “incomplete profile” option in my options area. Has this option been deleted?

My main question, however, has to do with monthly vs annual subscriptions. I’m sorry if this has been dealt with, but my search didn’t quite provide a clear answer. I suspect it’s come up a few times though.

Do I have to make these two separate subscriptions, or is there a way to have someone sign up and then choose either the monthly or annual price?

If not, then what do you propose as the best way to go about setting it up? If they are monthly and decide to change to annually, how do I make sure they’re not getting charged for both?