When working to ASME Section IX, position is not an essential variable provided notch toughness is not required when qualifying the WPS.
As you would expect, position is an essential variable when qualifying the welder. So, the welder can be qualified in any position provided the existing WPS is revised to permit welding in all positions, both grooves and fillets. The WPS should provide the limits for the groove angles, root openings, use of backing, back gouging, limits for the angle between members for groove welds, etc.
Good luck - Al