How to update the progress of a task which does not have resource assigned

Hello,

Your question is how to update the progress of this activity. OK.
So my question is how do you do to follow the progress before this modification of date ?
In fact, it seems difficult to calculate a progress without data workload. Alternatively, you can continue to use a rule of three to personally assess the progress calculated over different timescales.
Perhaps I don't fully understand your concerns so my answer at this point is : The progress depends on your own calculation then written down into a textual area.
Otherwise, use the functionnality "BASELINE" to keep a photo of your project with this activity you can compare later with your current Gantt. 
Best regards