From UA's website:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">As an added bonus for Premier members of United’s award-winning Mileage Plus program, upgrade clearance policies will increase to 48 hours before departure, from 24 hours. All other upgrade clearances will follow existing time lines. (effective first quarter 2003)</font>
Does anyone know when this change will actually happen?