You would file an amended return for 2018. The PMI deduction would lower your tax liability, which would reduce the amount of energy credit you received. You would get no additional refund from your 2018 return. but you would have a larger carry forward amount of credit for this year.
If you do this, you must prepare the amended 2018 return before filing your 2019 return so you know the correct amount of carry forward. Remember that your maximum savings will be 22% or 24% of the amount of PMI you paid, and you won't get any benefit from the deduction unless your total itemized deductions are more than the standard deduction of $12,000 for single or $24,000 for married filing jointly.
