It didn't replace, in my experience. I didn't know this was to be A Thing, and last week I was going through my settings and noticed the facebook.com address. "Huh, I guess they're adding email addresses to the existing URLs everybody has (or was able to sign up for those years ago)." My other (login) email address was still there, but as an alternate or whatever.

It replaced both of my public email addresses.

I bet it did if you click "Edit." I bet that your other email addresses are there and are marked as "Hide on Timeline."

Maybe something changed in the last, oh, 5 days?

Look closer. Your previously public non-facebook address is now set as hidden. People attempting to email you will be forced through Facebook. "Hide from Timeline" option will be chosen.