I could have sworn this came straight out of that thread we had. . .like two weeks ago? hold please I'll find a link
http://ecomodder.com/forum/showthrea...rsion-955.html
The long short answer is no.
The valve timing is part of it but the expansion stroke is longer than the compression stroke(the crank shaft has to change from what yours is(equiaxed)). Thats why atkinson is more efficient than otto buts its not quite so simple as a cam change.
Its why it never really took off. A little more FE for substantially more complex.