We have upgraded our application from Berkeley DB sleepycat version
3.3.11 to Berkeley DB sleepycat version 4.5.20.

Following are the changes in Berkeley DB 4.5 release:
The log file format changed in the Berkeley DB 4.5 release. No database
formats changed in the Berkeley DB 4.5 release.

So can anyone tell us is it required to upgrade existing old Berkeley
DB files to newer versions? If yes, then what all we need to do?

I assume you are using TDS.

1. Backup everything.

2. Force a checkpoint of the environment.

3. Run DB 3.3's recovery against the environment. This will remove
all meta-data which changes between versions.

4. Recreate the environment with DB 4.5, either with db_recover or
your application's recovery procedures.

5. Forcing a checkpoint is recommended.

This procedure is documented in more detail at:

<http://www.oracle.com/technology/documentation/berkeley-db/db/ref/upgrade/process.html>