A subscription transfer facility was announced few weeks ago, I found the initial message in this forum.
. We have the ability to reassign Pro apps to any of our Free apps. All we have to do is:
Go to the Billing Section, click the three dots next to the Pro app we wish to no longer be Pro, and click “Transfer subscription"
Select the app we wish to become Pro from the dropdown.
So, for apps with monthly subscription, transfer this subscription to another app. Then, upgrade the previously monthly pro app to an annual pro app using the coupon.
I have not tested by myself but it should app. On my side, I have first canceled the monthly pro app, but it also expires the 11th of June, so as cancelled, I can’t change anything for this app till expiration. So I have used the coupon to another app, a’d I will switch the 12th if June!