The lifters will "break in" to the old cam, and will have the old worn cam "imprint" on them, if the cam holds and doesn't get worn away by the new lifters that don't match it yet.... Then when you put a new cam in there later, the now already broken-in lifters will not match the new cam, and both will potentially be ruined. So... when you put the new cam in, use new lifters with it AGAIN.
The lifters can be taken apart and cleaned. I would do that rather than waste a new set of lifters.
As a sidenote, my #6 cam lobes were REALLY worn. Lost maybe half the lift. So yea, measure the lift before doing much else.
Jan
