Existing users subscribing with same email address

First of all - Excellent bit of software, works like a dream.
Quick question, how do I allow existing registered users to upgrade to subscribers. At the moment they find a brick wall if they want to use the same email address.
I feel sure there is an easy answer but for the life of me I cant see it.
Cheers
Mark

Forget the last question I have sorted it.

OK Mark. Thanks for the update. Please continue to setup and use the extension. If you need help, please don't hesitate to ask

Regards,

Tuan