Answer
Once an object has been placed on the canvas, the time unit may only be changed to a smaller time unit (e.g. days to hours, but not hours to days). This restriction is enforced because converting to a smaller unit is always unambiguous and will never result in changes to dates or durations. For example, two weeks is always 14 days or 336 hours, so there will be no changes to activity dates or durations. Converting to a larger time unit would involve either rounding or truncation and would potentially result in changes to dates and durations (which would result in altered gaps, floats, drifts, etc).