The most important thing to consider is whether or not there are valid justications for the effort of changing posts to the past tense, given that you are increasing your workload and increasing the risk of introducing errors into the text.
In terms of justifications, the only one I can think of is that readers may get confused reading something in the present tense that occured weeks or months ago. If the date of the piece is clearly and prominently displayed, then this problem can be mitigated quite simply, with no need to change the text. You could also introduce an italicized foreword of some sort explaining that this was written during the event in question. Even just a simple "This event has now finished" (which you could even automate to display) should make it absolutely clear to readers that the events mentioned were in the past.
It's worth mentioning that re-rediting the posts can create sticky situations trying to make your text work. For example, imagine there was a quote, something like:
The event is kicking off on August 20, 2013 and will continue through August 26, 2013. John Smith will be having an autograph session during the event. John Smith said, "I'm really looking forward to meeting the fans, and there will be a prize giveaway of my latest book."
So, now you have to edit the tense. You shouldn't change the quote, since changing the quote would change the words that were actually said. Do you then say something like, "Before the event, John Smith told us ..."? However, what about the prize giveaway? Do you delete that? Leave it in? If you leave it in, will it make sense? Won't readers be curious to know who won, since the text now reads as if it was written after the event?
In my view, it would be far better to simply leave the original post as is (perhaps introducing the "This event is now finished"), and have a follow-up piece written that reports back on the event. From a marketing perspective (assuming this is indexed by Google), this is a much better idea: you don't really want to be deleting or altering what is possibly well-indexed content, and adding new, fresh content is a good idea (making sure it's not simply duplicated content, of course). You can even link from the old, original event content to the new follow-up content e.g. "The event is now finished. Have a look at what happened when John Smith met his fans."
