Do get new money
Check out those fresh bills.
Go to the bank and exchange your random 100RMB bills for spanking new sets and (better still) sequentially numbered notes (the print order, if you didn't know). It's lucky to have a string of following numbers and if you're going to put money in a hongbao, make sure they're not your tattiest bills.
No one will reject the money because, well, it's money, but they'll be sure to notice how dirty your gift is.