Sorry if I'm reiterating known point, when the storage is full, API will stop working, so you won't be able to download files at all.

So you are completely stuck if you have too many files. Like I had. I used to keep pictures on onedrive, and used 6 user license. When the license expired, they locked me out completely. I couldn't download my own files! And the web UI is a crap.

So had to pay again for a year, this time I backed up all files locally.