We have a client that sent us a Cobra Payment check with the wrong date on it (9/25/06 vs 9/25/08). If we receive a letter from the client asking us to change the year of the date to 08 can we do this?

Do not alter the check. Proper thing to do would be to request a check with the proper date.
In practice, no one will ever look at the date on the check. If you deposited the check, there is a 99.99% chance that the check will be paid. The only person who could object would be the maker, and you already have a written statement that the date on the check was their mistake.