Good question. As the thread author you have the ability to add a poll after the fact (via the Thread Tools button), which I personally would prefer over a new thread to avoid the redundancy and retread. The software will pick up the poll just fine as it was designed to do.
is fearing that this will skew the results vs. the way the threads usually work, because the poll wouldn't have been there since the thread inception and wouldn't have gathered votes alongside the posting of the existing replies. She hence advocates new threads being opened instead. However, I think this can actually be mitigated by writing a new reply to the thread, announcing the addition of the poll. Between thread subscriptions and the relatively stable forum population making regular visits, I think this should encourage enough existing thread participants to return for a vote. Additionally, the mere existence of my site already changes the dynamics a bit by making older threads and their polls more visible. Not that voting ever required posting first anyway.
Furthermore, we were in agreement that the retro review thread project ought to be rate-limited, and I was planning to finally open my Harbinger
thread this weekend or Monday-ish or so, so with a new thread we'd have a minor scheduling conflict.
Bottom line, I have no authority, but I'd say add a poll to your existing thread and write a reply to announce it