how will this be done?

especially for cartoons.
like, is that even possible?

i watch the simpsons on FOXHD, and see no difference, except for more vivid colors, and somewhat sharper lines.

The same way it's done for movies. Shot on film or HD video, mastered in high-definition. Pretty much everything in prime time on the broadcast networks that's not animated, a news program, or a reality show (with even a few exceptions for those) is in high-definition.

Sure. A number of traditionally animated Disney films have aired in high-definition, for instance, and Voom has a channel called Animania that shows nothing but cartoons in HD.

The Simpsons isn't produced in high-definition. You're watching an upconvert.

Just because you're watching the "HD" version of a channel doesn't mean the program is in HD. Many times you're still watching the SD feed.