Using classic Outlook for Windows in business environments
Preserve Reply/Forward content when changing the From address in New Outlook
New Outlook doesn't work like Classic. Other than a clunky manual Copy/Paste there is no solution. All I can suggest is that you Vote for and Comment on this Feature request
Preserve Reply/Forward content when changing the From address in New Outlook
https://feedbackportal.microsoft.com/feedback/idea/ca02da50-66b2-f111-aad1-6045bd8232d6
(You must open the Feedback Portal link and "Sign In" at the top right of the Feedback page before you can vote).