Yes, you can change the subject, with three caveats:
- Don't edit the forwarded content (body of the forwarded messages)
- Don't eliminate important information like recipients of the prior e-mails, or timestamps (when the back and forth happened)
- Make sure that you state clearly why you are attaching the forwarded e-mail
Even if you edit the subject of your e-mail, the forwarded messages should still contain the original subjects they were sent with, and the original recipients. So long as you don't remove the identifying information (like sender, recipients, and times), and you don't edit the content of the e-mails you are forwarding, the person you forward it to won't lose any information even with a changed subject.
As Justin suggested, be sure that you make sure your body is clear about what you expect the person to do. To be safe, I would also suggest saying, "For reference, I've forwarded the discussion we had on this subject below" or something of the sort. This makes it doubly clear that the below messages are for reference only and weren't a conversation they were involved in.