Turning Pelvis into MMD lower body

Most humanoid models (FBX, VRM, etc.) have a bone called pelvis (also called hips or spine).

It works similar to MMD 腰 (back) bone, but 腰 is semi-standard bone that controls 下半身 (lower body) bone.

In order to convert to MMD models, you first have to turn pelvis into 下半身.

Pelvis bones usually exist at the top of the hierarchy. You should change spine (= 上半身, upper body) parent to the same as pelvis bone, so that upper body and lower body (originally pelvis bone) can rotate on their own.