Feature to downgrade RAW to jpeg, and delete raw file

In parallel, I save my pictures in a Cloud and therefore I'm limited in the total amount of space it takes.

So, keep your RAW files on a hard drive, an external one will do nicely.

Are you talking about Lightroom Classic (LrC icon), or Lightroom (Lr icon)?

If you are talking about Lightroom Classic:

Usually people will tell you it is a bad idea to delete the RAW images. I will tell you that as well.

Propose an option to copy the RAW files in a directory (but outside of LR) for safety. That would allow to to reimport it if case of selection error

Your photos are not INSIDE Lightroom Classic. They are simply stored on the hard disk somewhere. If you want to keep the RAW files, just leave them where they are — or have LrC move them to an external disk but keep them in the catalog. Why bother remove them from the catalog if you want to keep them and you might want to re-import them again??

Perhaps, you are trying to fix some problem that you haven't told us about — like LrC is slow — and so you have decided taht a smaller catalog would fix this problem. If that's what you are thinking, that is false ...  a smaller catalog will not fix speed problems in Lightroom Classic (exception: backups will happen faster). And if the problem is disk space (which you have mentioned), why ask about how to keep the RAWs? Keeping the RAWs doesn't save disk space at all.

I really see no benefit to what you want to do, if you still want to keep the RAWs. But, if you still want to go ahead:

First, select the RAW photos for which you want to do this.

Next, you can do an Export of the photos selected photos, and select the export option "Export to: Same folder as original photo" and "Add to this catalog". This will add the exported JPG to the catalog file.

Third step: with the RAW photos still selected, delete the RAW photos from the catalog — or — have LrC move them to some other disk and then delete them from the catalog.

Adding: if you are talking about Lightroom Classic and you are storing the photos in the Adobe cloud (you didn't say where in the cloud you were storing your photos), your RAWs are not stored in the cloud, and the smart previews that are stored in the cloud do NOT count against your allowed cloud disk space, so no need to clean that up.