What makes up the grey? Is it a swatch, a gradient or a blend?
If grey is a swatch, you can do the same change swatch to P662 and then lighten it.
If it's a gradient, you could open that gradient and substitute P662 for black and, if necessary, use lighter P662s for intermediate colors.
If it's a blend, you might try duplicating the file and then do another blend.
Then again, it you have something like a gradient or blend going form P662 to white and you don't know what its composed of, it might be just as easy to just keep the white and P662 and redo any intermediate steps.
Using OSX 10.3.9 on a G4 & G5