change cell entry value after macro executed

to avoid duplicating entries I wanted to remove the "marker" that controls it's inclusion in a macro (if a letter "m" is entered in column c:c, on filtering the range, associated data is transferred). Before exiting source workbook I wanted to remove all instances of "m" so next time they would excluded from the data transfer.

However it would be preferable to leave a marker in the source sheet for future reference

before exiting routine could the cell values in column c:c (if they equal "m") be changed to "mu" or something so that the marker remains but they will not be included in further filter occurances

I had

Range ("c:c").ClearContents
rng.Autofilter
End With
End Sub

as the end of my routine

any pointers appreciated