Yes, you will need to wait until your returns is either rejected or accepted BEFORE you can change your AGI. IT takes about 48 hours for you to know if it has been accepted or rejected.
If your return is rejected then you can login to your account and change the AGI and then refile your return. If you refile please check to see if that one was accepted. If it was rejected again then there are other errors on the return that will need to be corrected and then re-filed again.
When your return is rejected due to a incorrect 2018 AGI verification, does it take much longer to get "accepted" after one resubmits? I am going on 4 days. It usually only takes a day or two.
Being rejected once does not add any additional time on the return being processed and either accepted or rejected when you re-file. The IRS does not start processing returns until today, and they are processed in the order in which they are received, so I would count the 48 hours after today since today is the start of e-filing.