Where are you going in Japan?
What kind of riding do you do?
At 215lbs the 154 banana is not going to work in powder, even the 159 dark series is very small for a deep day at your weight.
My advice, keep the banana which you like to ride on non pow days and sell the dark series (or trade) for a proper powder deck. There are many to choose from.
Everyone will suggest different lengths but I would be thinking in at least the high sixties for Hokkaido, of course it depends on the type of board you go with.
You could easily ride a 170 or 180 birdman which some of the bigger forum members swear by. ETM and memphis hawk both of whom ride deep pow here in Japan and are comparable weights.
On the other hand if this is a short trip and you don't think you will be coming back regularly renting a pow deck on the deep days is an option.