No, you don't have to pay for multiple subscriptions.
You do not have to pay for multiple subscriptions if you upgrade/downgrade. If you upgrade from Cam Plus to Cam Unlimited you will be charged the full price on the day you upgrade and the existing amount remaining on your current subscription plan will be applied towards your next payment cycle.
Example 1: I have one Cam Plus annual license which renews on June 1. If I upgrade to Cam Unlimited Annual today in February, I will be charged $99 today. Next April, I will be charged $92 as my existing amount will be applied.
If you downgrade your plan from Cam Unlimited to Cam Plus , you will be charged today for the full price of Cam Plus. You will not be charged again until the existing amount is used.
Example 2: I have one Cam Unlimited annual plan which renews on August 1st. If downgrade to Cam Plus annual plan today, I will be charged $19.99 today in April. Next April, my existing balance from Cam Unlimited will be used I will be charged the prorated amount for renewal.
The amounts may vary based on the number of cameras you have Cam Plus licenses on.