ALV - Dynamic header?

I'm relatively new to SAP, and I'm currently polishing off a program based on an ALV display.

The spec requires three variables to be shown in the header, above the grid. Currently I have those implemented by way of a string concatenation in HANDLE_TOP_OF_PAGE.

The problem is that these variables can be changed by the user after the initial ALV display, meaning that the versions shown in the header are no longer correct. Right now the only way to make these update is to close the program and reopen it.

Is there a way to force the header to update and reprint the variable values, rather than just the grid itself?