The site can exist either on PC 1.0 or 2.0. We do this Product Catalog update to 2.0 within the live site. Plans and Addons of PC 1.0 will become items and item price points in PC 2.0.
So the other entities (Subscriptions, Invoices, etc.) in the live site will be unaffected and they will be linked to the corresponding price points in PC 2.0.
Yes, technically it is possible to take your site back to PC 1.0 since we retain the data structures of PC 1.0 and create new data structures for PC 2.0 during PC 2.0 upgrade for your live site. This fallback will be helpful if your deployed code changes don't work during the upgrade. We can switch your site back to PC 1.0 while you fix your code. Once you are tested and ready again, we can go for the upgrade: you can deploy the code changes and we upgrade your site to PC 2.0.