Technically, an amended return filed before the due date is a superseded return; however, without a checkbox I don't see any option in ATX but to do an amended, which will not be a quick process (regardless of e-filed or mailed in).
Just had an amended return rejected for e-filing--taxpayer filed as surviving spouse and original was e-filed. This was an IRS (not ATX) reject that the account had been frozen because primary taxpayer was deceased. Didn't see an alternative to mailing it in. Mail in, I'm guessing it makes a 4 month process about a 5 month process (on average).
Re-thinking this...since it was a mail-in, should have done a 1040 and wrote "Superseded" at the top of page 1.