OH. Darn.
Thankfully, I learned this on a server that’s in use only for testing.
For the other sites, should I just give them a fresh redis and throw away what was scheduled? Do a backup/restore?
FWIW, I’ve not noticed any issues with crosstalk in the past year or so. 
And there is a way to set the DB. .
EDIT: Well, the good news is that I can enter the container, edit redis.conf and restart it and it starts working again.
If you’ve got a hint on how to move a site from DISCOURSE_REDIS_DB: 12 on one redis container to another redis container, I’d love to hear. Or maybe just don’t care about scheduled jobs?