Best way to offer free trial for premium content?

I want to offer a one week free trial with full access to the site, and then after expiry give the option to upgrade to a paid account if the user wants to continue using the premium content.

The problem is, if a user attempts to login using an expired login, they are given no option to upgrade, they simply sees a message that says “Account is inactive”.

The member cannot then sign up for one of the paid options using the same username and email address, as the system detects that their details are already in use (even though it is an expired account).

Is there a way round this?

(Note: I haven’t yet set up any ‘paid’ levels, just testing things at the moment)