Your understanding is right, DB engine can't be changed later once RIs are purchased. reference document is here.
But, I'd suggest you to please log a support case to AWS under Account and billing category. Additionally, if you have Enterprise Support plan, you can talk to your account manager and technical manager and provide them the additional context.
They may check with Billing and Concierge team, if this request can be accommodated or not.
Hope this explanation helps.
Comment here if you have additional questions, happy to help.
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