In RTL, when a hero or Overlord uses threat/fatigue to upgrade black dice to silver/gold during combat, are they restricted by the campaign level in terms of how many dice they can upgrade?

No, they are not. Repeat after me, forum software: No, they are not.

Awesome, thanks. Thanks very much. Repeat after me, forum software... my message is just fine and doesn't need to be any longer. To be any longer would be to become ridiculous.