They could just continue to fly the Norway flag, but they also could end up with their house being torched as a result.
They could replace it with the Russian Naval Jack.
https://upload.wikimedia.org/wikipedia/commons/6/66/USSR%2C_Jack_and_fortress_flag_of_naval_fortresses_1924.svg
Or this one.
https://upload.wikimedia.org/wikipedia/commons/thumb/2/21/Naval_Jack_of_Russia.svg/1280px-Naval_Jack_of_Russia.svg.png