Tower Upgrade to 3.8 fails when PG Database exists

Dear Community

I have Tower 3.7.1 running and want to upgrade to 3.8 - there is an existing PG database running and the inventory file is configured as described here: https://docs.ansible.com/ansible-tower/latest/html/installandreference/upgrade_tower.html

But in each case, the playbook tries to install a new postgres database and ignores the existing database. Then the job fails:

TASK [postgres : Make sure PostgreSQL is started] *********************************************************************************************************************************
fatal: [rh8-tower.kestenholz.net]: FAILED! => {"changed": false, "msg": "Unable to start service postgresql: Job for postgresql.service failed because the control process exited with error code.\nSee \"systemctl status postgresql.service\" and \"journalctl -xe\" for details.\n"}

Do you have already updated an existing Tower installation? If yes, do you have an advise for me?

Regards, Martin