exemplum:
xy control g-zxcv
g-zxcv pass your message
xy control g-zxcv REQUEST IFR PICK UP , position radial/dme from navaids, altitude, intentions, destination.
ATC will come back with a new squak and instrutions and ifr cleareance.
if this change is due to change in wx conditions you should say
xy control g-zxcv UNABLE TO MAINTAIN VFR REQUEST IFR PICK UP , etc.. as above...