When should you let users toggle between and old and new version of a site or feature?
This is an appropriate strategy to use when the legacy version of what you’ve redesigned is supporting a high volume of business-critical traffic. It provides continuity for the “I like things the way they are” audience and gives them the option vs forcing them to change. Even if testing suggests people will adopt it, it’s better to make it optional as long as you can.
It also allows you to compare old version traffic vs new version traffic conversion paths via Google analytics or Visual Website Optimizer or some other analysis tool. Then you can see if there are obvious patterns or correlations and fine-tune your new design if you need.
A few years ago I helped update an e-commerce ordering UI that was supporting thousands of daily transactions. We were very careful about rolling the new version out and provided this “toggle legacy version” mechanism for users to opt out of the redesigned experience to use the old one.
We also tracked that toggle click as an event (“Go back to the classic site”) so that over time we could see fewer people reverting as more and more of them made the choice to just stick with the new design. Eventually when that number fell below a certain threshold, we were more comfortable with moving towards taking it offline altogether and “sunsetting” so we wouldn’t have to support 2 UIs.