In case your client currently has a perpetual license but wishes to switch over to Subscription Billing, you can follow the steps below:

1. Navigate to [Your Customers] then activate [Subscription] for the customer in question.
   
   
   
   
   
2. Navigate to [Setup]  [Customer Infrastructure] > [Managed Hosts] and click on the [License] icon showing 'Freeware', 'Standard', 'Unlimited' or 'Unlimited Plus' when hovered over (depending on your current license edition).
   
   
   
   
   
3. This will bring up a list of your licensed hosts, as per below. Simply click on Unassign and Subscription Billing will be assigned automatically.