Upgrade question, going from 8.3 to 9.1...

Folks-

We're in the process of upgrading our postgresql install from 8.3 to
9.1.2. Our main application is using postgresql-8.3-606.jdbc3.jar against
the existing postgresql database. (Java jdk1.6.0_29, Oracle's version, not
OpenJDK. Its in use by hibernate. (Other Java applications use the
database too, but mostly via hibernate or rbatis)

My questions are:
1) is it necessary to upgrade the jdbc jar file to the 9.1 version, or is
the 8.3 version 'upwards' compatible? If it is, can someone point out me to
what I would fail in apps that did not upgrade?
2) Even though we are not using features from the JDBC4 version, should we
still use the JDBC4 variant due to the 1.6 JDK version?
3) Is there any potential issues if an application is using the latest 9.1
JDBC driver against postgresql 8.3.x database?

Obviously, we're hoping not to require an updated jar file, and just delay
the upgrade until the next application release. But I need to know if
upgrading the JDBC driver with the database upgrade cannot be avoided.

Thanks

--
Virtually, Ned Wolpert
http://codehead.tumblr.com/
http://codeheadsystems.wordpress.com/

"Settle thy studies, Faustus, and begin..." --Marlowe