I’m in the same boat, often, and there is no solution. I do the original writing with Pandoc/Markdown, but I’ve resigned myself to working in the client’s form

I’m in the same boat, often, and there is no solution. I do the original writing with Pandoc/Markdown, but I’ve resigned myself to working in the client’s format for subsequent revisions. If a revision requires a lot of new content, I write that again in Pandoc/Markdown and paste in the converted result.