Hi,
For Unity licensing, you won't need to get a new Unity license. Unity only cares if the partner server is Exchange or Domino (not which version of either).
Regarding Exchange licenses, that depends on where you got Exchange in the first place-- if you bought it directly from Microsoft, then the question should be posed to Microsoft directly.
If Exchange was purchased directly from Cisco, then that would indicate that you obtained both your Exchange 2000 and Exchange 2003 software from Cisco (or a partner) and I would direct the question about Exchange licensing to whoever supplied you with the Exchange 2003 software.
I hope this helps.