Do some calculations using tax software to see if the Roth conversion would be beneficial even if the IRMAA adjustment was not approved. Treat the IRMAA as an additional tax. You might find that you're mom will still pay less in the long run by doing the Roth conversion even with the one-year increase in IRMAA. Filing MFJ, each tier is a difference of $50k to $60k in AGI and each tier represents an increase in IRMAA by about $1k. So a jump up by one tier works out to about a 2% increase in tax, but you also have to consider the possibility of some of the additional income falling in a higher income tax bracket so you would have to decide whether to target the upper end of an IRMAA tier or the upper end of the tax bracket (and do forget about other possible side effects of an increase in AGI such as Net Investment Income Tax). Also note that 2% is less than the generally 3% tax rate increase that is presently scheduled to occur in 2026 when the provisions of the Tax Cuts and Jobs Act of 2017 expire.
When targeting an AGI at the upper end of a tier, it might be good to leave a little bit of margin, particularly if there are variables that you might not know until after the end of the year.
