Is it possible from within composing a new message in Pmail to reverse a string of characters from Upper-Case to Lower-Case?

No. The only way I can think of to do it is to copy it to another program that can do such a thing and then replace it with the result. There are a number of online utilities that will do it. This is one: https://convertcase.net. It will not handle automatic capitalization of proper nouns though.