Can any body tell me how to change a Cancel button to Ok button after a process finished?
(maybe just change the caption of the button? I guess)

Thank you in advance!

Put one of the buttons on top of the other and use ShowWindow(SW_SHOW/SW_HIDE)
to show/hide the appropriate button.

This can be dangerous, if OnOK() and OnCancel() do different things...

It was sort of assumed that there is only one button and its handler, only the caption changes and the program is aware of the 'state'.