I recently got a third Xbox Live account and found out that this only worked with a guest account.
I'm posting this tutorial in the hopes that it will spur a new glitch for main accounts, but for now this will only work on guest accounts.

Found by: Im4eversmart


Requirements:
Three controllers
Two Xbox Live gold accounts

Controller assignments:
controller 1 = none
controller 2 = Higher prestige account
controller 3 = none

Tutorial (Prestige Glitch Method):
1. Press A on Xbox Live to join a multiplayer lobby with the higher prestige account using the second controller.
2. Sign in a guest using the first controller.
3. Press A on the store using the first controller.
4. Press B on the second controller.
5. Press A on the third controller. During the white box screen, which appears right after you press A with the third controller, press A on the second controller. Now you should be able to navigate your accounts using the second controller, and sign in the lower level account.
6. You will get kicked back to the main menu. Press A on Xbox Live with your second account to get in to the multiplayer lobby.
7. Press A on your first controller and it will be your guest, but it will have the prestige and rank of the higher prestige account. Custom classes are also transferred.

Notes:
As stated previously, this only works with guest accounts. I have tried and do not know if this can be ported to main accounts.
For some reason, this only works in this combination. If you try to do the swap starting with the higher level account on the first controller and the guest on the second controller, it will not work.
This glitch uses my account swap method, which can be found here.
Signing the guest account out and back in will cause it to become the original level.