I'm quite sure that Roku and Dish and HBO could figure out a way to handle something as simple as this. These newfangled computers can do a lot of good stuff.
Actually, Roku did that for me back in June. I had already paid for the next month when I found out about and signed up for the annual plan. I contacted Roku and they had already prorated my annual plan to reflect the $15.
Roku Customer Support:
"I understand from your message that you already switched your Monthly subscription to HBO Max Ad-Free Monthly into HBO Max Ad-Free Yearly. And your account were being twice charged for the same channel. Please be advised that the amount that was processed on your account for the yearly one was already prorated. We already less the amount $14.99 to the original price of the HBO Max Ad-Free Yearly."
What they did for you before wasn't refunding anything, it was just crediting your already-paid partially-used current monthly payment toward the new annual payment. I think I recall where HBO Max did the same thing for those subscribing and paying directly through them.
HBO Max's support site explicitly states "There is a no refund policy for subscriptions billed directly through WarnerMedia." For refunds from other providers, such as Roku, it states "Contact your billing provider with regard to any refund requests for charges." So maybe you'll get lucky with Roku and they'll cancel your HBO Max account and refund you the unused $125 for the remaining 10 months of service. Then you can sign back up through Dish.
help.hbomax.com
As for newfangled computers being able to transfer your account, again, there have to be systems and protocols in place for those computers at different companies to do that, so that Roku could send your prorated remaining annual payment over to Dish and have it be credited to your account there. I just don't think anything like that exists in their systems, because why would it? Kinda like returning an item to Walmart.com and asking them to credit the amount back to your Amazon account. Just doesn't happen.
EDIT: And here's Roku's stated policy on subscription refunds:
"All content purchases and subscriptions purchased outside of The Roku Channel are pre-paid, final, and non-refundable unless otherwise stated in the content and service provider's credit and refund policy."
All content and subscriptions purchased and managed directly by Roku are pre-paid, final, and non-refundable unless otherwise stated in the content and service
support.roku.com