You guys know that temperature is not the biggest factor for deciduous trees to come back from dormancy right? It is actually the sun that gives the signal. Sure warm weather might speed up things a little sometimes, but trees know it is time to start pushing growth by knowing how long the sun has been out there for a day. Where I live, I have found the best time to re-pot is around April 5th, march might work but it is early. Where I live, buds start coming out around April 5-15,and right now trees are starting to push new growth (May 2). So it depends on where you live. Just look for when buds come out, and do it then. A little early shouldn't kill the tree, and even a lot early for deciduous trees should be fine. But optimal time is when buds just come out. Forsythia are also a great indicator, the flower very early because they put out the buds on the fall. So when you start seeing the first Forsythia flowers, it is time to repot.