Oh, it certainly is possible. Read the body text. Respond to the body text.
The subject line is not part of the message. It is only a way of cataloging the message, and a way to keep the continuity of the thread. If the subject line doesn't seem to match what's being discussed in the body text, you can then go back and see how the discussion evolved into what it has.
Would it make sense to update the subject line? Occasionally. But then readers that use the subject line text instead of the index numbers and references to thread messages wouldn't necessarily be able to follow the evolution of a thread.
The bottom line is that if you are going to reply to a message, read the text of the message, and reply to the text of the message. It's actually pretty simple. And then you don't need to worry at all about how the original subject line relates to where the discussion currently is.