Prorate and extending CamPlus subscription

I have 2 cameras with 1 year CamPlus subscription. I have 2 additional cameras without subscription.

There’s a new offier for multi-camera discount for 4 cameras. What would be the best way to take advantage of the new plan?
If I purchase the new subscription will it extend the subscription the 2 cameras that already have subscriptions? Should I cancel the existing subscription for credit, then buy the new 4 CamPlus licenses?