Are actions and bonus actions interchangeable?

While I know you can’t cast 2 spells of first level or higher in a turn, and actions can’t just be used as bonus actions, is the reverse true?

Say there’s a barbarian cleric that rages as a bonus action: can they use healing word as an action, when it normally takes a bonus action? Can bonus actions be used as actions in the case of using a bonus for something else such as rage?