Update:

Got an email back from Start9. This is what they had to say:

"In changing Core to Knots you will have changed the RPC credentials. The services that depend on Bitcoin will need to have their configs opened and saved again (nothing more) so they can fetch the new credentials. Afterwards they should restart and work".

I'm not sure how to do this though. I can find the config and RPC settings of Bitcoin Knots, but how to have that communicate with the rest of the services (do I manually do this, or is there something to do with restarting)? I do not see an option in Electrs to input this information. See the photo.