If you forgot something on your return or need to change anything on your return for any reason, and you have already e-filed your return. Then do ***NOTHING*** at this time. You ***HAVE*** to wait.
If your return is rejected, then you can just add what you forgot and/or make any changes necessary and submit the return again by clicking that big orange TRANSMIT MY RETURNS NOW button. *DO* *NOT* *EVER* amend a rejected return. ***NEVER***.
If your return is accepted, then you still need to *WAIT* until the irs has processed your return completely and entirely. Your return is not processed in it's entirety until you have physical possession/control of your refund. Once your return has been accepted ***AND*** processed, then you can file an amended return.
Understand two things:
1) Any amended return can NOT be e-filed. The IRS says so. For an amended return you have to print it, sign it (both the 1040 and 1040X) and mail it to the IRS.
2) Understand that an "AMENDED" return is only to change a return that has been ACCEPTED by the IRS. If your return is NOT accepted and you click that AMEND button, the bottom line is, you are screwed and it's *YOUR* fault. The mess created can be fixed. But it will be a nightmare of having to call telephone support and being on hold for what could very easily be hours, during the tax filing season. You'll have to deal with hangups/disconnections and whole lot of other crap, and it's *YOUR* fault for clicking that AMEND button on a return that the IRS did NOT accept.
