Yes to both
Absolutely nothing wrong with putting up a brand new review for an old effect, or - indeed - in adding your thoughts to an existing review thread.
Perish the thought the the original reviewer should have the ultimate say on how good (or otherwise!!) an effect is.
In terms of responding to an existing Review Request, best bet would be to start a new Review thread for the effect, write your review, and then add a link to that on the 'Request' thread; all nice and neat then
Hope this helps!
Rob