is in being 表示在于,相当于lie in,用that宾从不同确切。改成is lying in being即可。
这是简单的主系表结构,介词短语作表语。其中
being superior to your previous self是动名词短语作介词in的宾语。
你的改写没有语法错误,但意思与原句有出入。
这句话写错了吧,正确如下:
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.