Questions on Upgrading PostgreSQL from 15.0 to 15.9 and Setting Up Streaming Replication

Dear PostgreSQL Community,

I have a production database setup with a primary server and a standby
server. The database is currently running on *PostgreSQL 15.0*, and I plan
to upgrade both servers to *15.9*.

I have the following questions regarding the upgrade and replication
process:

1.

*Upgrade and Replication Compatibility*:
- My plan is to perform a failover, promote the standby server
(currently 15.0) to primary, and then upgrade the old primary server to
version 15.9.
- After upgrading the old primary server to version 15.9, I want to
configure it as a standby server and set up streaming
replication with the
new primary server, which will still be running version 15.0.
- Is it possible to establish streaming replication between these two
versions (*15.0* as primary and *15.9* as standby)?
2.

*Efficient Replication Setup*:
- The production database is around *1TB in size*, and creating
replication using pg_basebackup is taking more than 2–3 hours to
complete.
- Is there an alternative method to set up replication without taking
a full backup of the entire cluster but instead using only the WAL files
that have changed on both servers?

Your guidance and recommendations on these questions will be greatly
appreciated.

Thank you for your time and support!

Best regards,

Subhash