Its official that Yahoo has been hacked, http://www.nytimes.com/2016/12/14/technology/yahoo-hack.html?_r=0, and it is time to make that change from Yahoo to another email address. For a free service, looks like Gmail is the best there is at this time.
My quick guide to reducing the migration pain is as follows:
Step 1: Start adding your Gmail to all correspondences and signatures, plus start giving it out instead of yahoo
Step 2: Setup your Gmail to start receiving email from your yahoo address see How to Access Yahoo! Mail in Gmail
Step 3: Respond to all your correspondences via Gmail
While the cut over is immediate, to get your correspondents will take some time to finally start using the new address, probably 3 to 6 months, so be patient