There's a threshold for the mandatory electronic filing of those returns and it's 10 or more returns.
There does not appear to be anything in the Regs at the link your cited that would require the electronic filing of an amended 1120-S return after an original 1120-S was e-filed. If that is in error, please point to the specific section.
Regardless, TurboTax Business (again) will not allow a user to e-file an amended 1120-S after an original 1120-S was e-filed with the software.
