Should there be a hyphen in 're-rate'?
If you have already rated something and then you want to rate it again, what is the correct term?
Rerate or Re-rate?
Both are possible, though re-rate is more transparent and thus easier on the reader. And of course you could simply say "change rating" or "rate again", as you just did.
Both are acceptable forms.
"Rerate" is acceptable by dictionaries. Click on the embedded hyperlink for evidence.
Though as @RegDwight stated, "re-rate" is easier to be read.