You should be able to keep it for now, but I'm sure that they'll force you out of it eventually. Just make sure that it doesn't fall off when you activate the new device. I would recommend activating it online, so that you will see any features that may change to ensure that the unlimited plan is not one that changes.
If the new device is unlocked to be programmed thru Verizon.. were you looking at replacing your current device with the new one or having both on your current plan? you have the option in March to continue service from what I can see under the Plans & Service tab above under your acct thur the link My Verizon.. and do a Prepaid Plan,,, (month to month) where you would not be on a 2yr contract as you stated your plan is now.. or you may renew services for both or one under a contracted plan.. Please note that there should be an activation fee involved at the time the new phone is activated ..
Hope that helps you with your question..