The bigger picture is here -- https://advice.thisoldhouse.com/showthread.php?t=121829 -- but one part of this is pouring new concrete piers to support a covered porch/deck.
Is it OK to dig around an existing pier -- and pour concrete around it to make the existing pier larger? Could I just dig with a clamshell digger and enlarge as I need? Do I need to clear out all around the existing 6" peir and sit a new 12" tube around that existing pier? Do I need to completely remove the existing pier? They are probably 24 to 36" deep.