ELB is now legacy. I would be very surprised if they add HTTP/2 support to it; it was one of ELBv2's big launch features.
Is that a difficult migration? Admittedly our stack is tens of servers, not tens of thousands, but at first glance it looked like I could achieve migration via a simple update to a cloudformation script.
I just spent the last week migrating our dozen micro-services over. It wasn't too bad, and resulted in less complication as you can now have one internal and one external lb rather than one per service (and less security groups, DNS records, etc.).
The only real difference is that you now have to configure routing and target groups.