Wine is correct. The 787-9 was the last fleet to arrive with non-Polaris seats. It only makes sense that they would be the last to get the new seats and this is what the plan appears to be. It's a real shame because these have the longest range of any of UA's fleet and they are used for the longest routes.
From what I last read, the plan was to start the 787s in Q4 of 2019. I take that as the first aircraft can enter MOD on 31DEC19. UA's goal is to have all MODS complete by the end of 2020, but I think that's wishful thinking and suspect Q2-Q3 of 2021 is more realistic.