Here is a script I wrote in Autohotkey that copies values from one Revit schedule to another. This is not done through Revit API. Instead this uses the crude Windows copy/paste commands.
It expects the cells to be contiguous. The cell values need to be instance driven (as opposed to type driven). Trying to copy similar “Mark” values for different instances might trip an “Elements have duplicate 'Number' values” error in Revit.