Our 2017 return was rejected because of incorrect AGI. Is it acceptable to "white out" the incorrect number and hand-write the correct AGI in its place -- and then mail in a copy of the corrected form?
It is never acceptable to "white-out" any information on any kind of official or legal documents. If you want to alter a document, mark a line through the incorrect information, write in the correct information above or next to it the correct information and initial the changes. The incorrect figure should be perceptible.
However, the previous years' AGI is for identification purposes of the IRS to allow you to e-file. Since you are not e-filing, it is not important.
The AGI that was incorrect was not the AGI for 2017. The 2017 AGI does not need to be corrected.