Are users going to be required to change their passcodes to 6 digits from 4 after upgrading to iOS 9 on Touch ID devices? Will they be prompted to make the change right after upgrading?
6-digit passcodes
I don't know about the change of length of passcodes after upgrading, but I can tell you that users are still able to set a 4/6/alphanumeric passcode when setting up their new iPhone or restoring a backup, even on a device with Touch ID.
When you try to set a new passcode, iOS 9 will prompt you to enter a 6-digit passcode instead of 4-digit one. However, it doesn’t force you to change your old 4 digit passcode, but if you do try to change it, you will have to use a 6-digit passcode.
You also have the option to continue using a 4-digit numeric code. You get the option by tapping on Password Options.
What I posted above applies to the final release version of 9.0 as well.