This is a design request, rather than a real bug, but it's very annoying. I just installed the latest nightly build and can't recall verbatim what the dialogue says but it's something like "Do you want to update the keychain references so they work in this Webkit". That is the question deserves a "yes or no" answer. However, the two buttons do not have labels reading Yes and No or even Yes and Cancel. They say "change all" and something else. This is extremely confusing and every time I update Webkit, which granted is only about once every four months, I have no idea if I am about to disable all my passwords from keychain. Please look into providing choices that match the question. I'm not a developer, just an average user and may be out in left field, but I do find it frustrating to have no idea what my choices are. It should read something like "Do you want to update your keychain references so all your favourite sites that require passwords continue to work or erase them all". Then give clear choices "update" and "erase", so I know exactly what I'm doing. As it is, i've no idea what "Change all" means.
This dialog is presented by Mac OS X, and is in no way related to WebKit. Please file a bug via <http://bugreport.apple.com> for Apple engineers working on Keychain to investigate it.