Before panicking about the weather, check the forecasts for Tokyo. It may be cold and snowy, or it may be nearly 10°C. Even so, a coat and sweater should be sufficient. I took students to Japan one January, and we were fine with cloth coats and sweaters. You're not going to be landing in Sapporo, after all. If worse comes to worse, there's always Uniqlo or Muji. I've used both stores to deal with sudden changes in weather, buying short sleeves in a warm October and long sleeves in a cold April.