> Turns out, it's usually better to be fast and break things
In terms of # of correct guesses, sure. In terms of damage from guessing incorrectly over time, usually not.
The trick is to separate processes - A CSS update probably isn't very risky, and an auth or login update probably is. Don't bundle them into "website updates".
Run ahead with the layout changes and spend some time on failure-planning for the auth change.