Cable
Omono
That's exactly what Ryan Neil talked about in one of his videos. He said to keep the shin (roots directly below the trunk) intact in the original soil and the put bonsai soil all around. The roots grow into the good soil for a few years and then when you go to repot the old soil just falls away and you grow a new shin. He recommends mounding bonsai soil in the new box/pot where the void is.