If the existing ePub is using serif throughout (either explicitly or by default), the easiest way to update it is to redefine serif to Arial and add this definition via Calibre's extra-css option. The simplest possible example is in my
post #3, but you can also define the four font variants (normal/bold, normal/italic) for serif (and sans-serif) the same way - just like post #1 but without the "body" entry.