Due to the fact that yahoo photos is closing on September 20, 2007, a three months free flickr pro account is provided as an incentive for users to move photos from yahoo photos to flickr. It applies to both pro and free filckr account users. An extra 3 months pro account will be topped up the old one once all your photos have been imported from Yahoo Photos account into your Flickr account.
Login to your yahoo account and make sure at least one photo is in your yahoo photos for transfer then go to http://closing.photos.yahoo.com/ and follow the instruction step by step for the switch.