With bittering hops there is very, very little difference in flavor... most people wouldn't notice if they drank both side by side. The only thing I would be concerned about is if your IBU/AAU are correct. Cluster and Challenger tend to fall in the same range, so they should be close, but I would make sure and adjust accordingly. Honestly, if anything, using challenger would be more appropriate since it is a British hop and cluster is American.
"Why did you.... what was the point of... how drunk were you when you decided this was a good idea?" - DMartin