Short Stories Turned Into Books?

I've noticed that some books, like Fahrenheit 451 and others, either began as short stories or had parts of them published in magazines. While I understand that every book has to stand on merit, what would be a modern agent's reaction if a query letter mentioned this? Let's assume the contract with a reputable magazine expired, the rights revert back to the author, and he/she expands it into a book. Would previous publication help or hinder?

Aggy B. said:

As long as you have the rights it should be a non-issue. Novels are so much longer and more complex than short stories that it's really hard to make a comparison between the original and the expanded project.

Now, if the short story were really well received or won awards or something you would likely want to mention that to an agent, but I'm not sure it would be something you should feel compelled to mention otherwise.

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Of course. Rights aside, I'm thinking if it were published in a very well-known magazine like Analog or Clarkesworld, something that has produced award-winning stuff before.