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…
I was very reluctant to switch from Yahoo to Gmail in the first few years of Gmail. But eventually I did it, and couldn't believe how much better Gmail was at spam filtering than Yahoo - like an order of magnitude better. I never looked back. Recently I stopped using Yahoo Messenger, too, because they've been adding a lot of nonsense to it lately.
When Gmail hit general availability, the quality of filters was not what average users expected. Things would get flagged simply because Grandma was not in the trusted circle of silicon valley.
Eventually they bought Postini, both to boost corporate email business, but also to source a spam filter that had been battle tested across large consumer mail installs.