Yes you will have to cover your body as well that is exposed if your going lighter. At this pint you can't go local with your foundation and you want something that will last longer on the skin.
You also don't want to go with cream for the body as it's easily smudged off at a convention with the amount of people bumping into you or you into them during the day.
You want to go with water activated cake makeup. and Use a makeups sealing spray to set them for the day. (all these brands make one of the sealers as well.)
The best work I've seen in turning yourself another skin tone (outside of Cloud Atlas) is DistyoHunters Witch look.
hes take you through building color layers which is hard to do but necessary when creating a "new skin" for yourself human skin is not flat which is why foundation can look so wrong when changing your skin tone.
For me I gave up the thinking of I have to go darker because I'm so pale all foundation look Fake 1 or 4 shades darker on me. My skin is a blank cansave and any subtle nuances I have in my skin are lost and I look really Orange/yellow/or pink with darker foundation on.
On a random note I could kick myself for not picking up this issue during IMATS this weekend.