Here's an outline of what I've done so far:
* Used to have LR5 catalog on a NAS drive
* CC refused to migrate said catalog because it was too old
* Installed Classic trial version and tried to upgrade catalog
* Classic refused to migrate since catalog was on a network drive
* Copied catalog to a USB drive (of course there were corrupt files which threw off Finder)
* Finally started CC, stated migration
* CC refused to migrate because there were too little disk space available
* After realizing I won't be able to free up enough space (have 128GB Macbook Pro) I see it's possible to change location in CC to a custom location. Set this to a new folder on the same USB drive as before.
So, the problem now is that CC starts migrating and then fails saying "Not enough space on main volume" - but I asked it to use an external disk.
Below is a screenshot of my Lightroom CC settings. Notice that the top paragraph mentions space on "Disken" (my internal disk) and below it says the location in use is "File Disk" (my USB drive).