Club designer Ralph Maltby does annual analyses of iron model clubheads to come up with each model's Maltby Playability Factor. MPF gauges the user-friendliness of the clubhead using a 6-variable equation; one variable is VCOG.
For the Rocketballz irons, the VCOG = 0.782". At 6% lower, the Rocketbladez would have VCOG = 0.735", or about 0.047" lower. The VCOG is just one factor for the clubhead, and does not even address the effect of the shaft in the club.
If you are hitting the Rocketballz decently, what would you gain by moving to RocketBladez? Usually moving from last year's to this year's model doesn't deliver that much gain, unless there's some techno earthquake at play.
I would suggest you the try RBZ and the RBladez side by side and see if there's a difference in performance. That's why we have launch monitors. If you bought the clubs online from a wholesaler, the problem you have is you don't get to test them first.
A TM fitter can measure for what you need, and then order the clubs from the factory to your specs. Unless you get alligator-skin grips and a super-upgrade shafts, this wouldn't cost you any more than buying off the shelf in the store.
(You may want to cancel your order until you do a side-by-side test of the two models).