Oh, I see how you did it.
But to avoid getting a surprise big bill, (see jdpuglisi above) maybe you can activate the centro on your 700P line, temporarily, then after a while re-activate the 700P and deactivate the Centro??
I had to temporarily re-activate my old Treo 650 while waiting for my Centro replacement screen to arrive. Those sneeky guys at Verizon took away my cheaper 30 dollar Centro data plan and re-instituted the expensive 45 dollar a month one. Once the Centro was fixed, they deactivated the 650 and reactivated the Centro, and I supposedly got my cheeper data plan back.
So for you, I guess you are paying for the expensive data plan for your 700P?