While you can't revise feedback you left Ono your own, you can ask your seller to send you a feedback revision request. Contact your seller and explain that you would like to change the negative feedback you left him to positive and could he please send you a feedback revision request. Once he sends the request, agree to it and you will be able to change the negative feedback to positive and revise the DSR ratings you left.
the seller has 30 days from the time you left the negative feedback to request the revision.
if the seller doesn't know how to send a feedback revision request, send him this link.
http://pages.ebay.ca/help/feedback/revision-request.html