Think of a 401k or an IRA like an umbrella. Anything done under the umbrella is a nontaxable event (except Roth conversion). Once it leaves the cover of the umbrella it gets hit with taxes.

Just because it has no taxable event doesn’t necessarily make it a good idea though. “Trading” in the 401k will lead to less profits more often than not. If this is part of a purposeful change in asset allocation that’s fine. But if you think you can time the market, think again.