Undo 'Action-> Change the current user & the current date/time'

I have in one of my forms a dynamic rule that 'timestamps' a date and user field when a checkbox field  is flagged.
Works nice :-)

It might sound unlogical but in my situation I would like to undo these timestamps (empty them) when a certain condition is met.
- In a perfect world the emtpy trigger should be when another person is selected from a drop downbox
- as an alternative I already whould be happy when the fields are emtied when we unflag the checkbox field again. (like toggling between two statuses)
I cannot find a way in dynamic form to empty a field based on a change in another field.
Any workarounds or options I simply missed??



(for whom is interrested:
This has to deal with that projects in preparation bounce for certain preparing actions between contacts that provide certain information for a project before it is eventually assigned to a PM.
This bouncing can happen a variable amount of times in any random direction- it's like controlling an Q&A process and trace where the action needs to go and who the current owner of te action='project-in-startup-phase'  is).

Thanks!