bcrypt isn't reversible (with respect to the password). The point is that Blowfish(key1, "message") is not at all similar to Blowfish(key2, "message"). This is called resistance to related-key attacks (look this up for a formal definition), and is an explicit design goal of every real-world block cipher.

bcrypt is just Blowfish(key-derived-from-salt-and-password, "OrpheanBeholderScryDoubt"), so it inherits this property.

It's not reversible, but block encryption should be easy to compare for similarity. If the old password is 5 blocks, and 4 of 5 blocks match the new password (in order)... I'm guessing this is what the gp was talking about. (of course block size matters, if all passwords are a single block that doesn't help)

That's why we use a fresh salt for each password. Bcrypt incorporates a fresh salt into each new hash. Your and the GP's concern in this area is not warranted.