that is a problem.....
suggest calling the IRS AFTER you know that your tax return was filed and accepted and talk to them about this
you may get lucky as depending on when the IRS decides to use 2018 tax returns instead of 2019, it wouldn't impact your stimulus payment now
here is the worst case scenario - you resolve it with your 2020 tax return. What is not well advertised is that what is really occurring is that these stimulus payments are really an ESTIMATE of what you are really entitled to.
next year, you'll calculate what you are ACTUALLY entitled to based on the 2020 tax return (income, dependents,etc).
if the ACTUAL is greater then the estimate (which is your worst case), then you'll get the $1200 rebated to you at that time
if the ESTIMATE is greater than the actual, you won't be required to give the money back to the IRS.