FYI: You cannot add a 72G drive to a rootvg built on 18G's.
The original install would have set the PP size on the 18G's to get as close to 1016 PPs per PV as possible, and a 72G would have way more than that per drive if added via extendvg.
The last time I tried to do a chvg on a rootvg to adjust the factors that control that,
AIX (5.3) laughed at me and said go away. (I was doing the exact same thing only with virtual hdisks served up by a vio server).
mksysb or clean install. Personally I favor the latter as mksysbs are almost always messy.