I'm really sorry to say, but: too late. Literally 99% of those I know used to be a Yahoo Mail user (Y! Messenger still biggish in Romania, though loosing ground quickly to social networks), but a big part of them are using GMail now. Big, ugly ads and an "anti"-spam engine that makes SpamAssassin look good; what's there to come back to? Plus everyone has a smartphone/tablet now and what's that without a Google/GMail…
http://www.campaignmonitor.com/resources/will-it-work/email-...
Yahoo Mail is still more popular than GMail.