My server hardware died a few months ago, and I “promoted” the mirror. The resulting configuration is that the server (master) and mirror (slave) swapped roles. Does this affect the license? Do I need to swap licenses as well?

Nope.

The license renewal refers to one license as the mirror license, but the system using that license is now the main server. Does it actually matter, or can I just renew the same Nxxxxxx license on each system even though I swapped their roles?

It doesn't matter. The actual renewal is always for the master, that one will update both licenses.
   
ChrisP.

Thanks!