Maybe someone would find it useful. I have written my step taken to migrate from Derby testing environment to DB2 which allowed as to have more then 10 users:

The main problem was that DB2 has to be installed on a remote server and I had just ssh shell to connect to it.

1. download DB2 and the licence
Start here
http://www-01.ibm.com/software/data/db2/9/download.html
and download the DB2 file and licence.

2. install under linux ssh shell
Unpack the file under a directory with enough free space. I took steps described here:
http://snippets.dzone.com/posts/show/3682
http://www.fduran.com/blog/db2-linux-installation-notes/

3. remember to enable TCP/IP connectivity

look at page 78:
http://www.redbooks.ibm.com/redbooks/pdfs/sg246899.pdf

4. make RTC aware of the new db it needs to connect to:
You have a very handy web UI wizard to do this task.
https://yourServersAddress.com/jazz/setup
and follow the steps.
You can also do it manually, by changing the teamserver.properties file, which can be found in /jazz/server/ directory.
The most tricky part is to give a proper db location properties set up correctly:

# Comment out above lines, uncomment the following three lines and customize example location to use DB2

com.ibm.team.repository.db.vendor = DB2

com.ibm.team.repository.db.jdbc.location=//localhost:50001/JAZZ:user=db2inst1;password={password};

com.ibm.team.repository.db.jdbc.password=put_your_password_here

These lines are the same in the webUI interface.
After fulfilling them in the UI wizard, you can choose to test your db connection. It should return green thick :-)

5. enjoy!
Now you can enjoy the new db running under your RTC. In case you would need to backup your data from Derby, check here: https://jazz.net/help/rational-team-concert/1.0/index.jsp?topic=/com.ibm.team.install.doc/topics/t_migrate_derby_db2.html

Hope this helped,
Regards
Blazej

Post scriptum:
You can find this also here:
http://jezzyk.blogspot.com/2009/05/rational-team-concert-migrating-from.html