At a high level, the DBA will need to handle the same basic duties -- that is, managing performance, managing change, managing security, managing backup and recovery and assuring availability. So the core skills you learn as a SQL Server DBA will serve you well as a DB2 DBA. Of course, the devil is in the details, though, and there is no quick answer to your question about "functional differences." You will need to spend time with DB2 to learn how it operates and how it differs from SQL Server. Off the top of my head, I would recommend that you start by focusing on SQL (there is no Transact-SQL equivalent in DB2), and then investigate the system differences (e.g. logging, buffering, file usage, etc.).
There may be external issues, too. For example, you may need to learn a new operating system for DB2. Yes, DB2 runs on Windows, but it also runs on Unix, Linux, OS/400, and mainframe platforms - SQL Server runs only on Windows.
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