Files overwritten when installing v9.6 in machine with v8.4 previously running

Hi again,

I previously asked in another list about what version would be the best
to make a pg_upgrade from v8.4. I've finally chosen 9.6 to make version
jump less drastic, though I've been testing pg_upgrade whole process
with both 9.6 and 10. But after reinstalling my test machine several
times now I have 9.6 so that's what I'll be talking about.

The problem I have is that when I install PG9.6 in a machine with CentOS
and PG8.4 (version that comes with OS) then v8.4 becomes unavailable.
Finally I've found that psql binary changes after v9.6 installation. But
checking files that are installed with this version, I can't see what
package causes the problem. And as I'm not used to *nix systems, I'm not
sure how can I solve this. The process I've done:

- Installed a new machine with CentOS installation CD, which installs
CentOS and PG8.4

- Made InitDB, started service (with "service postgresql start"
instruction).

- Without changing pg_hba.conf (nor postgresql.conf) file, I've
connected with psql, created some roles and some empty databases.
Disconnected from database ("\q") and restored some backups with psql
command.

- After all this, I can connect to v8.4 with root user and postgres
user. Finally I've stopped postgresql (service stop).

- Tried to start postgres again but using pg_ctl. Tested connection with
psql again (ok) and stopped.

- v8.4 is installed in /usr/bin and /var/lib/pgsql folders, that seemed
quite generic for me, so I made a copy of both (with "cp -pr" command):
/usr/bin ==> /usr/bin_8.4
/var/lib/pgsl ==> /var/lib/pgsl_8.4

- I've tested copy: started with pg_ctl (the one that was copied to
/usr/bin_8.4) pointing to copied data folder (/var/lib/pgsl_8.4/data).
Started ok. Tried to connect with psql (ok) and stopped again.

- At this point, I've followed
https://www.postgresql.org/download/linux/rdhat/ instructions to install
v9.6: download repo, install client and install server.

- Made InitDB in v9.6 and changed listening port in postgresql.conf
(v9.6 config file)

- After this installation, if I make "service postgresql start" I can
see "something" running (I can list processes with ps -ef) but when
trying to connect with psql it won't connect exiting with a message like
(sorry, it's my translation as original message is not in english):
psql: could not connect to server: file or directory does not exist.
    Is server locally running and listening at Unix domain socket
«/var/run/postgresql/.s.PGSQL.5432»?
Also "/usr/bin/psql" file is different than it's copy at bin_8.4 (some
Kbytes bigger than the original file).

Older PG version is now unavailable, and I can't run pg_upgrade to
migrate to a newer version. I've managed to make the whole process in
Ubuntu 18, but Production server is CentOS 6.8. Any clue about why is
this happening and how can I deal with this situation? During CentOS
installation I can't (or haven't seen) change installation directory for
Postgres. I now about data_directory in postgresql.conf, but not sure
how to manage binaries dir.

I have recorded all the commands I've run, just in case anyone need more
detailed info.

Thanks in advance,

Ekaterina