I've noticed that when Google and Facebook roll out a new interface they often either allow users to opt out of the new look for a brief period or they delay the release but allow users to opt in to the new interface. As an example Google Docs recently changed and gives this notification:
Google seems to be using this to collect data on why people prefer the old layout, as this notification now pops up if you switch:
I'm wondering what the practical reasons are for allowing users to hold back on changes like this. Is it just an opportunity to survey why users do or don't like a change (Google often takes advantage of this possibility) or does it actually allow users to better adapt to what would otherwise be a sudden change? Does it actually lessen the blow or does it simply put it off when users decide not to use a new interface?
I'm not talking about interfaces that allow you to chose between "new" and "classic" interfaces forever (like how Windows 7 allows you to select a "classic" look), but rather when there is only a grace period where you can chose which interface, after which you are forced to use the new interface. In web design in particular the "old layout" option appears to fade away quickly.