I am comfortable on all the three major databases and well know platforms (Linux, Unix, Windows)... I started as a Oracle DBA then as per industry/client specific needs, I expanded my horizon to other databases as well.
Since you are familar with Architecture, Tuning process etc - I think it will not be too difficult to pick up DB2 at all. The task for a dba remains the same .. I recommend that you start with reading db2 manuals/guides on the following topics: architecture, backup/reco (re-read if you are a production dba...), new features, administration and tuning. There are plenty of resources available online or otherwise that can help you to acquire knowledge.
Let me know if you have any specific questions.