When should I amend my Michigan return?
Amend your Michigan return only after your original return or claim has been filed and processed. If you are claiming a refund on your amended return, you must file it within four years of the due date of the original return (including approved extensions). For example, to amend a 2015 return that was not extended, Form MI-1040X-12 must be postmarked by April 15, 2020. (tax year 2019 due date extended to July 15, 2020)
If a change on your federal return affects your Adjusted Gross Income (AGI) and/or your exemptions, you must file an amended Michigan return within 16 weeks of the change. Include a copy of your amended federal return with all applicable schedules and supporting documents. Submit payment of any tax and interest due.