Our MAGI for 2016 was quite a bit less than for 2015. Last mnonth I filled out form SSA-44, expecting that there would be a retroactive adjustment for the already-paid 2016 premiums.
Not so. After a bit of digging, I found the relevant part of POMS:
https://secure.ssa.gov/poms.nsf/lnx/0601120005 section B.
It appears that they will only do a retroactive adjustment for a previous year if the 'life changing event' took place in the last 3 months of that year. So what I should have done is submit SSA-44 immediately the 'life changing event' occurred, which in our case was July 2016.
We are getting reduced premiums for 2017, so it's not all bad. But now we'll have to wait until 2018 to get a refund for the 2016 IRMAAs.