1-2 of 2 Answers
Should not hinder your current plan. You will probably accrue an activation fee for getting it set up, so don't be surprised when you get your bill.
I believe in order to retain the $70 plan you would need to swap out the SIM cards and on;y have the 1 cellphone device on your plan. You will still have to pay the $40 activation fee for them changing the IMEI /ESN from your "old phone" to the "new phone".