What you are talking about is this 100/50 USD bill:
I have had this problem many times. If you end up with old USD bills older that series 2003-6 then it won't be accepted by most countries and money changers outside the United States. More frequently the problem I have noticed is with the bills with the front face photo to be smaller than the current bills. Bills with the enlarged pictures i.e the new ones are widely accepted.
Note: This does not apply to the smaller denomination such as 1, 5, 10 as these denominations still use the smaller pictures.
I would suggest two solutions. The first is to try a large bank within the city you are traveling to. These have sophisticated machines and UV scanners which would allow them to check your money without concern and exchange it. Unfortunately this method is not fool proof and you may get rejected in one or two places before you come forth one which will accept it.
They are still legal tender and will be valid in the United States. They are not used outside due to lack of security within these old bills. So obviously the other solution is to either spend them in the United States or deposit them in an ATM in the United States where they are easily accepted. You can then withdraw from the ATM smaller bills which will be fresh and would most likely not have this issue.