The answer depends on your calling plan. If you are still on your former Alltel calling plan, then yes you simply need to activate the old device which can be done right here on the website as well as suspending your current device as lost.
Simply log into My Verizon, click the My Services tab, click ont he left where it says My Phone, and you should see options for suspending the line as lost, which should report the device to the company to be added to a watch list, then you can activate your old phone in the same place. This will ensure if somebody finds your device that they cannot use it without your permission on your account.
HOWEVER, if you have a Verizon calling plan, you will not be able to activate your old device, as it is not compatible with VZW calling plans. Regardless it's still a good idea to suspend the lost phone.