in tax year 2023 we did a Roth IRA rollover from one brokerage to another. In 2026 we determined that the rollover was mis-characterized as regular IRA.

How was it reported on the original 2023 return? The 3 year statute for 2023 returns ends April 15, 2027 (October 15, 2027 if they had an extension). 8606 can be filed as a stand alone form.

You need the brokage to now classify it as a ROTH, and when you receive the 1099R in 2027 fill out 8606 with an explanation explaining what happened. Don’t think you need to amend 2023 unless you took a deduction for a Traditional IRA, and like Susan said you have time to amend 2023 if needed

“but i believe the year-end 1099-R  would describe it a taxable income”

Well, you’re never taxed twice on the same funds.

That wouldn’t be a problem. That original rollover is tracked and documented as it is Basis. If it stays there until distributions begin, every distribution will be pro rata taxable, not the full amount. The Basis would be divided by the total account value, to determine percent not taxable. That amount then will be fed back into the carryover Basis tracking and reduce remaining Basis for the next distribution.

Essentially, post-tax funds rolled into the pre-tax account, the same as if someone was intending to do a Backdoor conversion. If the brokerage can unwind this and recharacterize the original rollover as Roth IRA, that would be best, of course. I’m pretty sure any earnings attributable to the Roth IRA amount stay as pre-tax, unfortunately. You should research that part.