I'm not sure about the difference between these two, like in the sentence "The boy regarded her "as" ... or "The boy regarded her "to be", is the meaning any different or can they be used interchangeably?

I would say they can often be interchanged in many contexts.

However, in other contexts, you could also interpret them this way.

The boy regarded her as his mother. He treated her as his mother, eg loved her, asked her advice, etc.

The boy regarded her to be his mother. He recognized the fact that she was his mother.

eg He said 'That woman is my mother'.