OK, they may have changed things. Response to current FAQ on second reviews is, "Yes, we allow members to submit reviews of multiple experiences with a particular location. <snip>If you would like to make a change to an existing review about a location, please contact us and request that your old review be removed. Once we remove it, we will let you know that it is safe to submit an updated review."

It may be that earlier rules required that the initial review be deleted regardless of circumstances- or else I mis-read them. I published an updated review a few years ago and both the original and the update were highly complimentary, but I'd thought that you had to ask to have the original review deleted first even though there was no change in the overall tone.