Postgres Upgrade from 8.4 to 9.1

Hi,

I am planning a Postgres migration from 8.4 to 9.1 to be able to leverage
the replication features available in the 9.1 version. I would like to
understand the following things in this regard:

1. Any good documentation which should help in this upgrade.

2. To be able to replicate the complete steps in a test environment
before doing it in LIVE which is running 9.0, is it possible to revert this
database to 8.4 and then upgrade to 9.1.

3. Any known issues and changes required to be done in the application
for this upgrade.

Thanks,
rajiv