Like the others say, kinda hard to know specifically what you're talking about without more details about the current bike and cranks and what you're considering. Likely the cassette is fine (or is it worn?).

One slight correction to Mulveyr's comments on Hollowtech. Hollowtech referred originally to the crank arm construction. Hollowtech I cranks didn't come with the bottom bracket (but you did need to buy the type and spindle length of cartridge Octalink bottom bracket that matched). Hollowtech II is the current generation of external bottom bracket type cranks and the bottom bracket bearings do come with the crank (as the spindle is incorporated into the crank itself). Shimano probably should have come up with a different name when they went to the external type (but it really referred to the crank arm construction so it did make some sense). Probably more than you wanted to know....