A Catholic who becomes an Evangelical I'd argue has never understood his Catholicism. There are millions of evangelicals who say "I was a Catholic until I was 15 and then I met Jesus." Or "I was a Catholic until I was 17 and then I became a Christian." Or "I was a Catholic until I was 20 and then I was saved." It's enough to make you want to visit their parish priests and say "What the heck were you telling these people?"
There are so many of these folks who say "Don't talk to me about Catholicism because they never taught me a thing. Now I've met Jesus!" By all means, finding one's relationship with Christ is the most important thing, but schlepping off to a Catholic mass was nothing and they found themselves in a non-denominational youth group or Campus Crusade. I'd say a Catholic who becomes an evangelical has never drawn on the riches of the Catholic faith.
Whereas an Evangelical who becomes a Catholic has taken the best of Evangelicalism and gone on with it to its fullest, reading their way into the Catholic faith, and becoming more evangelical and more of a Bible Christian than they ever have been.