Have you tried engaging the AWS customer service team on this? AWS has a public face that looks 100% robotic and automated, but I assure you there are AMAZING support & engineering teams behind the scenes who have a strong vested interest to make sure customers are successful using AWS.
IMHO, it might be worth a shot to send them a ticket that basically says, we bought RI's for the instance type we needed at the time, but now you guys released the new High IO type, and for us to be successful we really need to upgrade these RI's. So, the ideal outcome is you go buy your new reservations and then they give a prorated credit for the rest of your RI term on the old ones. I would imagine they might still steer you to the RI marketplace now but it's worth a shot if you can be persuasive enough.