I think all these polls should be tossed out and replaced with new ones. The problem is that we don't have enough options. We only have 5 options when really, we should have 10 options (i.e., 10 stars).
If I want to vote in between, I cannot. Not all these options fit. A middle option would fit better in a lot of cases. So instead of 5 stars, let's move to 10 stars. It's easy enough to create new polls with 10 stars instead of 5. Granted, it would mean starting over, but it would also mean a lot more accurate results.
What could be done is new threads created and then the moderators can merge the old thread in the new thread without the original first post and then delete the original threads. That would work very well and once done, we'd be all set.
If it's good enough for Amazon and Goodreads, it should be good enough for us. And the validity of several years of ratings to compare with is much more valuable than the added "accuracy" (which, if we're talking about opinions, is kind of a poor concept anyway; you're really talking about "granularity", which isn't the same thing).