Conversion of the Sakhalin’s network from 1067 mm to 1520 mm gauge would be pity for Japan's rail fans. That's because similarity between Japan's rail and Sakhalin's one is disappearing.
But, it's also a chance to add new trains for 1520 mm gauge. So, they can be looking forward to seeing new rolling stocks.
And also, if a plan to connect Hokkaido and Sakhalin with rail, railways on the island would be much more interesting.