If you make a decision to move from Lightroom Classic to Lightroom (the Cloud-Based service) this will ultimately result in all your Original Photos being uploaded to the Adobe Creative Cloud storage and all the info that would have been saved to the LR Classic Catalog file will also reside in the cloud storage. You will no longer have need of the preview files.
There is a process to do the migration but it requires a lot of thought and preparation and will require a lot of HDD space on your physical system.
I do not know if it is possible to import the original files directly from your Dropbox, this will also need some research.
However, when the process has been completed successfully you will be able to remove the image files from your computer system and send to an archive if you wish to do so.
The link below will provide additional info to begin the process.
Migrate photos and videos from Lightroom Classic to Lightroom.