Actually IRS says you may need to file an amended return if the reduction in AGI from the unemployment insurance exclusion makes you newly eligible for a tax credit that was not included on the original return based on the original AGI calculation. Here's the exact wording from IRS:
"There is no need for taxpayers to file an amended return unless the calculations make the taxpayer newly eligible for additional federal credits and deductions not already included on the original tax return.
For example, the IRS can adjust returns for those taxpayers who claimed the Earned Income Tax Credit (EITC) and, because the exclusion changed the income level, may now be eligible for an increase in the EITC amount which may result in a larger refund. However, taxpayers would have to file an amended return if they did not originally claim the EITC or other credits but now are eligible because the exclusion changed their income."
Another example might be claiming a retirement savers credit that was not claimed on the original return because of AGI at the time of filing.
