If you were only a resident of Michigan for January 2020, then you only owe MI tax on income earned that month. Once you became a resident of Texas, then any income you made for the rest of 2020 is considered as earned in Texas. Income on the sale of stocks , bonds, mutual funds, etc. is sourced to the state were you resided at the date of the sale.
Answer the Personal Info questions correctly and the program will direct you to file a part-year resident return for Mi, only, since Texas does not have an income tax.
It doesn't make any difference what address your brokerage firm has, although you should tell them to change it to Texas.
What makes you a resident of a state (or country)?
Generally, you're a resident of a state (or country) if you intend to either stay there permanently, or return there after a temporary absence. It's where home is – where you come back to after being away on vacation, business trip, overseas or out-of-state employment, or school. Many factors are considered, not the least of which are where you are registered to vote, own homestead property and are licensed to drive.
Residency rules vary from state to state. For example, if you spend more than a certain number of days in some states, you're considered a resident whether it makes sense to you or not. If you have any concerns, check with your State Department of Revenue for specific residency rules, especially as they apply to your particular situation.
