transition from LR classic to LR cloud

As Jim says there is a dedicated process to do this that works fairly well for most people. You just need a very large internal hard disk that can hold twice the amount of space that your entire library takes, or if you are using external hard disks to hold your images currently, you need an internal startup disk that has at least enough free space to hold ALL your files. This is a real problem for many people who hope that the cloud nature of Lightroom Cloudy can solve their hard disk space issues. Also, make sure you think it through very well before you jump. Lightroom Cloud misses many or most essential features that you might be relying on such as printing, virtual copies, hierarchical keywords, publishing manager and plugins, images that are only local, and much much more. It is also a very poor solution if you deal with 100's to 1000's of images per shoot especially if you are not on gigabit fiber internet. Many people switch and discover that many features are missing that they absolutely need. If you only ever deal with images online and are happy with all your images residing in the cloud principally and really don't deal with many images at a time anyway then Lightroom Cloudy might be perfect for you.

Reconsidered answer. Wish there was a way to remove answers.

If you are actually thinking of switching completely from Classic to the cloud, i.e. no longer using Classic after the switch, there is a purpose-built process for doing this. Open the Lightroom desktop app and use the File Menu>Migrate From option, then select the Lightroom Classic catalog. The migration process will then import all the Classic images, including all metadata and edits previously applied in Classic, and then upload all the images to the cloud.

See this Adobe Help document for full details, especially how to set things up before you start the migration: https://helpx.adobe.com/ca/lightroom-cc/using/migrate-to-lightroom-cc.html