On February 1, 2016, I dropped from Platinum to Peon on account of "life stuff" in 2015 (I am pleased to say I clawed my way back to Gold in 2016 and Platinum in 2017 where I have remained since). But in mid-January of 2016, while still Platinum (albeit a "lame duck" one), I bought a domestic economy class ticket for March of 2016, and was able to select seats in Comfort Plus for all four flight segments shortly after booking. Those seat selections "stuck", though I suspect that had the itinerary been subject to a change of equipment, it would not have.

On a side note, I did try to find the silver lining of status loss. One was no longer stressing about where I was on the upgrade list, and not wondering whether I should eat an airport meal or not as departure time neared and I was #2 with 2 seats remaining in F.