I think you will need to tinker with the CSS. You *could* try to replace the font files in the epub with the files you want and name them the same. Otherwise you need to adjust the CSS. Personally when I change fonts, I use a different CSS which I include in the general CSS. That way editing is easier.
Personally I use the tweak_epub
program to edit only parts of an epub. I find it ease to use. It is small but very useful.