There's probably no way to convert the legacy ext4+LUKS filesystem over, so, I guess I'll decide when F33 comes out whether I want BTRFS enough to do a clean install rather than an upgrade.
The btrfs-convert tool hypothetically leaves the ext filesystem all but untouched, and COW's the needed filesystem metadata onto the end of it, with data modifications coming thereafter (or intelligently stored within the free space of the ext system). You wait until you feel comfortable with BTRFS, then delete the preserved ext system and run a balance, which rewrites all data to disk in the usual structure. Alternatively, the preserved system can be restored, although I don't actually see instructions for that.
https://btrfs.wiki.kernel.org/index.php/Conversion_from_Ext3
I love btrfs and would trust it but am glad to have backups of irreplaceable data.
The wiki also doesn't mention how well this process plays with LUKS, so, given how sensitive headers and such can be, I'll probably wait to hear from some F33 early-adopters on the conversion tool to see how it turns out. I'm reasonably happy with my ext4 system as it is, and most of my core data is on backup disks anyway, so I'm not sure how much the integrity checking of BTRFS would help. Seems like it would be more critical to have on the backup disks to make sure I'm not propagating the bit-rot.