Problem when upgrade PostgreSQL 7.3.3 to PostgreSQL 8.0.3

Dear all,

I have web application which is run using PostgreSQL 7.3.3 and JDBC Driver pg3jdbc3.jar.
But when I try to update to PostgreSQL 8.0.3 and using JDBC Driver postgresql-8.0-314.jdbc3.jar,
I got error messages:
1. Cannot change transaction isolation level in the middle of a transaction.
    --> Does changing transaction isolation level in the middle of a transaction no longer supported?
2. ERROR: operator does not exist: bigint / character varying
    --> I don't understand this error message because all my query statements runs well in the shell.
         But when I call my query statement, I got the message.

What should I do to fix these error?

Best Regards,
Xin Xin