When should you let users toggle between an old and new version of a site or feature?

I've noticed a few sites that allow the user to toggle between a new and old version of a feature or site like Google used on their Forms redesign. We are currently trying to figure out how to let users play with both an old version of a feature that they are used to and let them explore a completely new way of completing the same task. This will be for mobile. We are considering using a toggle at the top of the screen to let the user explore their two options to complete the task.

Has anyone let their users have access to two versions?