http://pages.ebay.com/help/sell/questions/revise-item.html
There should be no problem with you contacting your own bidders. You just can't contact other seller's bidders in an attempt to lure them away from (or otherwise disrupt) their auction. In your case, I would definitely contact your bidders to give them the new info to avoid problems later, or give them a chance to cancel their bids, in addition to adding to your description.
