What would be a reason for replacing a working valve for a new rough-in valve in the shower. The new shower head and lever handle I bought says that a new rough-in valve is required. Is it really or are they trying to sell me something I don't need?

Does the new faucet match the old faucet? Same name, same thread patterns?

does the new faucet match the old faucet? same name, same thread patterns?

The old shower set is a Delta (9 years old) and the new is a Kohler.

The parts are not interchangeable. Replace the valve body.