IRMAA in the second year after retirement

Hoping someone can help me understand how IRMAA works in the second year after retirement. I understand that IRMAA typically uses a two-year lookback period based on your Modified Adjusted Gross Income (MAGI). I also know that if you experience a LCE, such as retirement, you can file an SSA-44 form to request that your Medicare premiums be based on your current year's MAGI.

My main question is about the subsequent years. Consider the following example for a married couple filing jointly (MFJ):

• 2025 Income: $400,000
• 2026 Income: $400,000 (Last year of employment)
• 2027 Income: $50,000. An SSA-44 is filed due to retirement (a LCE). Medicare premiums for 2027 would therefore be assessed based on the 2027 MAGI and the 2027 IRMAA thresholds. Any planned Roth conversions in 2027 should account for these 2027 IRMAA thresholds.

Here's where my uncertainty lies regarding the years that follow:

• 2028: How are Medicare premiums assessed? Would another SSA-44 need to be filed, again citing retirement as the LCE, and requesting an adjustment using the 2027 tax return? 2028 Medicare premiums would then be based on the 2028 MAGI and 2028 IRMAA thresholds? Consequently, Roth conversions planned for 2028 should consider the 2028 IRMAA thresholds?
• 2029: Medicare premiums for this year would be based on the 2027 AGI. If so, any Roth conversions performed in 2027 would impact 2029 Medicare premiums.
• 2030: Medicare premiums for this year would be based on the 2028 AGI. If so, Roth conversions performed in 2028 would impact 2030 Medicare premiums.

Thanks in advance for any insights!