I havent experienced quite a signicant jump in office here at work, but I wouldnt expect any issues as the upgrade usually just replaces the old files with new files. Only issue I can with your users is with Outlook and configurations and such. Not sure if the personal folders keep that same structure and such.
We are currently in the same process as far as upgrading from NT4.0 machines to XP Pro machines how many do you plan to roll out? Is this due to the fact that Microsoft is no longer supporting the NT4.0 OS? Thats the main reason we are doing it so long as we get the budget approval from our corporate office