Switch raid mode without rebalance?

Hi,

I recently created a new btrfs on two disks - one 6TB, one 2TB - for temporary backup purposes.
It apparently defaulted to raid0 for data, and I didn't realize at the time that this would become a problem.
Now the 2TB is almost full, and df tells me I only have about 200GB of free space. Which makes sense, because raid0 spreads the data evenly on all disks.
However, I know that btrfs can have different raid modes on the same filesystem at the same time. So I was hoping I could just tell it to "switch to single mode for all new data", but I don't have a clue how to do that. I *could* rebalance, of course, but is that really necessary? How does btrfs find out which raid mode to use when writing new data?

Gert