Upgrading side by side in Gentoo

I have separate production server running

"PostgreSQL 8.1.4 on i686-pc-linux-gnu, compiled by GCC
i686-pc-linux-gnu-gcc (GCC) 3.4.6 (Gentoo 3.4.6-r1, ssp-3.4.5-1.0,
pie-8.7.9)"

and want to upgrade this to something newer.
I can allow some hours of downtime for upgrade at night.

I have found Gentoo upgrade instructions here
http://hasno.info/2008/7/31/upgrading-to-postgresql-8-3-on-gentoo

I have no idea how PostgreSql is installed in this server, maybe it is
compiled from source.
So I'm afraid that using those instructions may corrupt the server so it
will no longer come up in morning and I'm not able to fix it due to lack of
allowed downtime and knowledge.

So I'm planning the following strategy:

1. Create backup copy in this 8.1.4
2. Stop postmaster
3. Install latest version of PostgreSql to separate directory
4. Restore to it from backup
5. If something fails, stop new and re-start old 8.1.4 postmaster.

The difficult point for me is (3).
How to install latest version in gentoo side-by side so that old version is
not touched?
In windows it is easy: I can select other directory from installer. How to
do same thing in this Gentoo ? I need also adminpack to read log files from
pgAdmin.

Andrus.