Change subject line in existing conversation before sending reply

I had a Conversation which was an e-mail trail prompting me to contact a prospective new client. They were in China so needed to be e-mailed. I wanted to e-mail from the conversation to keep it all in one place, but I was unable to change the subject line. I needed something more client facing so they weren’t seeing the original subject line from the e-mail. Fin said it wasn’t possible for me to change it.