If spring is on its way at your place repotting now is good. When cold weather is forcasted try waiting as long as possible but repot before vigorous growth begins. Make substrate and pot ready. Read about repotting. There are lots of videos out there. Live bonsai- Mirai the video on creating beech forest is a good one. But lot of others and several posts on repotting on here. Look for rootbase from top. Remove part of below and part of the sides of the soil. Elms cam be bare rooted. For your comfort leave most feeder roots and remove some thick far away from trunk.