Writing an email to a professor for teaching assistantship for the second time
This is a highly personal opinion due to an individual's emailing habits, but I would not attach pre-existing email on something more than a month old.  If you talk to the professor about the opportunity in the future, just add relevant information to the current email.  No need to add the entire backstory, unless it has important information that cannot be conveyed otherwise.

"Hello, I had previously inquired about such and such back in the Fall and nothing came of it.  I was wondering about any potential such and such for the upcoming semester."