Two production lines exist - RS-68 (Delta-IV) and RS-25 (STS & SLS). 1 is actively building (RS-68) and 1 is primed for production (RS-25).
I do not believe Rocketdyne is anything like "primed" for RS-25E.
RS-25D can restart construction in the meantime, all that RS-25E would be is a more economic expendable version to decrease carrying costs of the core engine. The J-2x was only in the J-2 family in name only, pretty much was a brand new engine. RS-25E will basically still function as the current engine, but changes will be made based on availability of funds and potential savings.