Is there a way of entering progress to a historic progress period?

Hi,

I am trying to enter progress for a project where I need to adjust percentages in a progress period which goes back in time.

There are more progress periods after this.

I have two columns, one is Actual % complete and the other is Planned % complete.

However when I select one of the progress periods the Planned % complete changes but the Actual % complete stays the same as the progress entered in the latest progress period.

How do I overcome this?

The reason for this is because I am trying to produce an S curve but I have a flat line for the first half of the project and I just want this first half to match the baseline.

Thanks