没错，非限制性定语从句有时形式上是从句，实质上其功用相当于一个并列的分句。

首先要明确的是，非限制性定语从句转化为并列句时，完全可以用并列连词but来改写。

至于非限制性定语从句转化为并列句时，是and来改写还是用but来改写，完全取决于句子意思的需要。下面是用but改写的例子：

He said he had no money, which (=but it) was not true. 他说他没有钱，但这不是实话。

We tried to develop plans for them to get along, which (=but they) all failed miserably.  我们试图计划让他们好好相处，但均以惨败告终。

He had a few relationships at college, most of which (=but most of them) were fairly short-lived.  他在大学时谈过几次恋爱，但大都十分短暂。

The search will not begin until dawn, by which time (=but by that time) it will probably be too late. 搜索等到黎明才开始，但到那时很可能会太晚了。