It's a question that's come up in a thread on the U2U, which I took part in. Given that Adobe don't appear to have any "back-door" method of moving the cloud-based assets from one account to another whilst retaining the assets in the cloud, we came up with a couple of suggestions.....both of which require downloading the entire contents of the old account and uploading them to the other, so not ideal. However:
1. You could use the Downloader (assuming it's still working correctly, though I have my doubts!) to download all the assets related to the old account to a local hard drive, which then would need to be imported (and thus uploaded) into the new account. The major downside (apart from the time it would take) is that all the existing album organisation would be lost, as it's not stored in XMP.
2. The method I personally would use, in the event I needed to, would be to create a new Classic catalog and enable it to sync with the old account. That would act like the downloader, but the big advantage is that the existing cloud albums would be recreated on download into collections. Once completed, that catalog could be migrated back into Lightroom desktop running under the new account, and all the collections would be retained as albums in the new account.
Neither is ideal, of course, but given time it would get the job done to some degree.