A：What is it?
A：That's not unusual.
A：She no doubt would have preferred that you take the time to paraphrase her message in the body of your reply.
A：To paraphrase means to rephrase or restate what someone else has said or written.
A：In that case, instead of hitting "Reply" and responding, "Yes, I am available;"
you should write, "Yes, I am available to meet on Wednesday at 11 AM to discuss the new product."
A：Yes. By restating her message, you have confirmed that you will be at the meeting on the right day at the right time.
A：Yes, In that case you can delete the original at the bottom of your response.
A：She is definitely right. It is very confusing to receive a message from someone who changed the subject in the e-mail and not in the subject line.
A：If I send you a message about a project we are working on and I use "Project A Summary" as the subject line, you have a clear expectation about what information is contained in the e-mail.
A：Right. Now let's say that I need to send you a message about our next team meeting. If I use your reply to me - which you recall has the subject line, "Project Summary"; you expect to read more about the project.
A：Better yet, if you are sending e-mail on a new topic, start a new message with a new subject header.