If command sponsorship is denied to Korea because of EFMP, can you call branch and request reassignment?

You can try, but the response will likely be that you will proceed unaccompanied for a year and leave your family where there is a support network for their needs. Korea is always one of the must fill billets.

If you requested the assignment through reenlistment, you can renegotiate your contract.
If you were placed on assignment by your branch manager, Korea is an unaccompanied tour and command sponsorship is granted by exception, not by default. Korea is frequently used for people with EFMP because you can leave for a year, reset your time on station and your family doesn't have to move.
You can always call your branch and ask, but that's probably going to be a strong no

YOu can request all you like. The answer, however, will probably not thrill you.