Any gotchas with changing IP address of Postgres Master?
There are several places you'd want to change the IP address for a PostgreSQL primary server. Most of them are actually outside of PostgreSQL itself or its configuration, though.
You already mentioned the pg_hba.conf so that doesn't need to be mentioned.

Tables that you have inet types for, or other columns you store IP addresses in. If you found IP addresses in $PGDATA/base via a grep, then that probably means that you have IP address entries in your tables, and looking at the information_schema.columns table for those would be most helpful.
Really, needing to touch anything inside of a PostgreSQL data directory outside of .conf files is not recommended at all.

postgresql.conf listen_addresses entries.

Any recovery.conf files that have the primary's IP address referenced for streaming replication.

IP addresses listed in pgbouncer or pgpool-II configurations.

IPTables or other firewall rules on your servers.

Application configurations.

Remote backup scripts.


These are just highlights.
A comprehensive list will require you going patiently through everything in your setup. Creating a good checklist of what needs to be changed in the event of a failover (and keeping it up to date!) will help you immensely in the future, and improve your disaster recovery and high-availability readiness.
Hope that helps. =)