upgrading marketpress lite to marketpress premium

I have marketpress lite installed. I have made some customizations to the templates file.

I need to upgrade to marketpress premium. I was really hoping it would be an easy process.

I clicked on to buy the $19 plugin and then realized I was billed $79 for full membership .... wtf??

How do I simply upgrade my marketpress lite to the full version.

I tried installing the full version and got an error. I assume it is because marketpress lite is still active.

I don't want to mess up the customizations I already made.

Please advise.

By the way ... for $79 per month you should really have a live chat option for support.

Thanks,
Michael