iphone won't let me use my old 4 digit passcode

A few days ago, iphone forced me to change my passcode. I don't remember why, but I changed it from say 1234 to 7890 . Now I want to change it back to 1234, and it won't let me.

inb4 "don't use a 4 digit pin"

Now I Can use a different 4 digit pin, eg: 0000. I get a slightly different warning message here. Note the USE ANYWAY

I'd like to know how to change my Passcode to the previous version. Bonus points if you know why I was prompted to change my password, or why I can choose some 4 digit passcodes, but not others.