I must convert a Latin9 Pg-cluster (Version 9.1) in utf-8 with minimal downtime, if possible. My idea is to use WAL replication for that. Is it usable for that or in other words, can WAL Replication handle different encoding on master/slave?
Short answer: no!
Is there perhaps an other way to do that?
I never tried it, but you can use some logical replication tool like Slony.
Is it possible to use on Master Version 9.1 and on Slave 9.6 (it will get the new master in the next step)?
Unfortunately no because on 9.1 version we don't have support for logical replication yet. We start supporting it since 9.4 using pglogical extension. As I said before you should consider use one external replication tool like Slony to do this job.
In a scenario like this it might also be worth upgrading to 9.6 using pg_upgrade while maintaining the latin9 encoding, and then do a second migratino which changes the encoding from 9.6 w latin9 to 9.6 w utf8. That way you can get access to the newer tools such as pglogical to help you do that.
