pgpool, pgmaster and pgslave migration to ubuntu 18.04

Hi,

I'm exploring the options to migrate postgresql master, slave along with
pgpool from ubuntu14.04 to 18.04. Please help me understand the following.

1. What are the available options for migrating from one version of O/S to
the other (Here ubuntu 14.04 to 18.04) ?
2. Document references.
3. Any best practices.
4. I'm assuming that migrating pgpool and pgmaster,slave can be done
separately and there isn't any dependency between them. (I mean first I can
separately migrate pgpool and then later migrate pgmaster/slave) is that
correct?

As far as Pgpool-II concerns, that's generally true. I don't know what
version of Pgpool-II you are using, and trying to migrate to but if
you are using Pgpool-II dedicated extensions, be sure to upgrade them
as well. For more information regarding Pgpool-II version up, please
refer to the release notes sections.

https://pgpool.net/mediawiki/index.php/Documentation

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp