I believe you will have to upgrade to LR Classic (The latest version of the Photography Plan)
The photography plan has both Classic and LRCC (and Photoshop). Once you get your catalogue on the latest version of LR Classic, you will prorobably have to upgrade your LRCC storage. Then you can migrate to LRCC. Then you can drop the Photography plan and just have LRCC.
I hope someone can suggest an easier path.