Debian squeeze version of postgres is 8.4, but I clearly remember having enabled some backward compatibility options for OpenAcs5.5, so it must really be something on my end.
Standard server root for packaged openacs in debian-like systems is /usr/share/openacs, which felt weird to me... I haven't really respected the debian convention, just put myself in a more familiar situation.
Another thing Debian did in the package and I replicated was the shell script. user and group are the ones the server is executed by. My replication was quite blind in this regard, just wanted to stick to the old package's style. Of course it is not necessary.
Debian doesn't install upstart by default, but sysvinit. Honestly tough, this is not the reason why I prepared the init.d script, I am just more used to it and wanted to feel at home...
In the end, as the data in previous site was not much, I decided to go for a clean install rather than upgrade. As I wanted to name the instance in a different way than oacs-HEAD, after openacs was installed I created another database for the instance and modified the configs of the new instance accordingly.
Thanks a lot for patch and hints!