How to allow users to upgrade from basic to advanced without paying too much

Hi :slight_smile:

How do I allow a user on my site to upgrade from basic business level to advanced business level, without having to pay £59.99 on top of their current £39.99 subscription?

Please see Link removed at members request

I'd like users to be able to pay the remainder ie £20 if they just want to jump up to the next level. How can I put this into practise please?