Although many taxpayers equate "amending" with "changing," the word "amend" has a very specific, narrowly-defined meaning in the world of taxes:
Amend (verb) – To make changes to an e-filed return that has either been accepted by the government or a paper return that has already been mailed. An amendment must be filed on a specific tax form created exclusively for that purpose (for example, Form 1040X is used to amend federal 1040 returns).
According to this definition, making changes to a return that hasn't been accepted or mailed yet isn't amending. For this reason, you can't amend an e-filed return that's in pending or rejected status.