你可以分写两个句子来理解一下,
He is no longer a theif.
He used to be a thief.
如果对第二句中的a thief 提问的话,应该问what did he use to be?而不是 who did he use to be?
然后尝试合并一下这两个句子,就得到了He is no longer what he used to be.
而He is no longer the person who/that heused to be.这一句其实是一个表语从句中套有一个宾语从句,the person是表语,而后面的who/that heused to be是the person的定语从句.
而The question is who will travel with me to Beijing tomorrow.这个句子之所以看起来很像你写的那个句子,其实不然,这个句子的主语是question,也就是说问题是,是什么呢,就是who 引导的那个问句.举个例子来说,比如上课老师问你这样一个问题：“who will travel with me to Beijing tomorrow.”然而你没有挺清楚,你可以问你的同桌：“what's the question?"这时你同桌给你的答案就是The question is who will travel with me to Beijing tomorrow.
看起来你对各种从句还不是很清楚,可以找一些不同的从句自己多分析分析,理解一下.