I keep all of my LR Classic original photos in a tree structure on an external SSD. All of my collections in LRC are synced to corresponding folders in LR CC.
However, only a few of my synced LR CC photos are stored as originals in the cloud. Most of them are in the cloud only as smart previews. I can find no way to force LRC or LR CC to sync the originals to the cloud.
Two options are to either drag and drop the Originals into Lightroom Desktop (Cloud), this will overwrite the Smsrt Previews, keeping the edits. The other is to Migrate the whole catalog, although this a big decision. Adobe recommend that you don't use both in unison, although some do. Plus there are some things that don't sync between Classic and Cloud, like Keywords.
It's probably best to have a think about where you do the majority of your 'work' before proceeding with a Migration.
Also, you'll need a Plan that includes enough Cloud Storage.