Here's an extract of the chat with Crashplan support (this was when I thought I'd uploaded more than 5TB):
You can still migrate, but we won’t be able to move the entire archive to the small business platform, we’re only able to transfer 5 TB of data.
So you have two choices really - we can deselect files from the archive and put it through maintenance, and transfer over a portion of that old archive.
Or we can simply start up a new backup. Given the size of your archive, we’d probably want to try to salvage what we can.
What does "deselect files from the archive and put it through maintenance, and transfer over a portion of that old archive" mean?
So the first step would be to tell CrashPlan to stop backing up certain directories.
https://support.crashplan.com/Configuring/Changing_The_File_Selection We’d need the total file selection to drop below 5 TB of data.