Hi, I need you help. My new bonsai ficus started to wilting leaves. They loose all strength and are very soft to touch compared to other leaves. Then they start to curl and turn black-brown. The twig they grow on is also wrinkled.

The tree is on the south-east window, there is winter so it's pretty cloudy during the day. Maybe not enough light?

I have read around and I think it might be an overwatering issue? What should I do to help it?

It is in the soil it came with from shop, I think it's not that good of a soil for a bonsai. I have some new pots and bonsai soil with better drainage, would it help to repot it with this new soil or would it kill it as it's winter?

Thank you very much for your help. Appreciate it.

Maybe this film can answer your question:

While i do agree with tropfrog that some leaves on the ficus will drop in winter, and it is good to see that video.

But i think you snapped the branch with the wirering, so there is no pressure going to the tip and leaves, and they start to wilt.