Please just have a profile option that says "Don't ever change anything on the interface", ever.
If you move the blue button labeled "Compose" located in the upper left corner of the screen to the upper center of the screen, my dad won't be able to find it and he will call me and say his email is broken. If you change the color of the button, he will call me and tell me that email is broken. If you change the label from "Compose" to "New Email", he will call me and say his email is broken. If you pop up a great big dialog box on the middle of the screen that uses a bold blinking font and uses very noticeable colors, and this dialog box says "Welcome to the new mail interface, click here to learn about it.", my dad will somehow figure out how to close the dialog without reading it or the associated help and of course, he will think that email (or the Internet itself) is broken.
No, I can't just teach my dad to be more flexible. Unlike other compatibility issues as technology progresses, I can not replace or "upgrade" my dad. He is 78 years old and is not into learning new tricks. He is a smart guy and is capable off learning new things, but he is old and crotchety and complains a lot every time he has to...
Please please please remember that there is a segment of the user base that views even simple interface changes as a huge deal.