Unclear how to reuse the keystore if it exists (register_rln.sh exits)

[s-tikhomirov]

I have a keystore from an existing RLN setup. I want to register a new RLN membership for the newly deployed RLN contract. On running register_rln.sh, I get:

$ ./register_rln.sh 
keystore/keystore.json already exists. Use it instead of creating a new one.
Exiting

From this message, it is unclear how to use the existing key store to register a new membership (or, more generally, how to complete the task of creating an RLN membership).