I assume you're using textedit or something like that to change the LabelOnLeftClick line in step.rc. After that do something like !execute [!reload][!labelrefresh] - !reload tells the litestep core to reload step.rc. !labelrefresh makes the label reread its (updated) configuration.
If you want a purely !bang driven solution (ie. without textedit) I suppose there could be a !changeclick !bang added to subclass.dll.