I'm pretty sure moderators can already change polls. I remember a Staff Complaint Tank made a while ago about Kyure where he included a poll ("Do you think Kyure is a bad staff member"), and just before the thread was closed the poll was changed to something like "Do you think Tank is dumb"

Some access to that functionality could be pretty cool though. You see a lot of polls on clan threads because there was a clan related thing that needed to be discussed and there was no other home for that poll, and I'm sure this could help in getting rid of those eyesores.