Tim, are you sure your answer?
Juergen upgrade farm to estate in 1 move, in second move he wants to builde 2 flats.
I play, like this:
1) after build estate this farmers goes to estate.
2) on end of the year, i can move this farmeres to farm
3) in next year i can build flat
Move farmers from improvments i always move to farm, but in the end of year.
Given that Tim designed the game, I'm fairly sure his answer is correct