As far as I'm aware there is no easy option to do that as the licenses are OS specific.
You could try contacting sales to see if they can convert and send one for a mac and for the remaining duration.
- Try the following:
https://support.avast.com/support/tickets/new, click the Billing option as it is possibly loosely related. From the next window, select the Order Modifications (again loosely related). You will need to fill in the information related to your current license, Name, Order ID, Billing Address and then the information and request to switch to a mac license.
As an avast user not an employee I don't know if this is even an option, but if you don't try you won't know.
