Removing the old 8x version would remove alot of modules dependant (i.e. httpd, php). I’m sure I could hack my way through it but it would be ugly. And I don’t see postgres 9.1 in the “bleeding edge” repository.
Unfortunately, there’s no supported way to do that. On CentOS 6, the only officially supported Postgres version is 8.4 (the one that comes with it).
Is it possible to install version 9.1 next to version 8.4? If so, you could just disable the 8.4 Postgres process, and install version 9.1 onto your system in addition to 8.4. That would satisfy the dependencies.