240V outlet to 120V, adapter?
 

I have a NEMA 10-30 240V outlet running my current washer/dryer stacked unit, which I plan to replace with a combo unit that uses a standard 120V outlet. Is there a converter or adapter I can use without needing to change the outlet, so that the 240V is available if needed in the future?

Nope. Best bet is to run a new circuit and receptacle and leave the old one alone.
If you are set against that you have to change the receptacle AND circuit breaker, AND the way it's wired.