Windows 10 preview for phones will likely have a rollback feature
Talking about the update process to Windows 10 on phones, Microsoft's Gabriel Aul mentioned on Twitter that "the ability to rollback if you hit an issue is very important to us." While this isn't a confirmation per se, Microsoft is definitely considering letting you revert to your older Windows build if you install Windows 10 and things somehow go awfully wrong.
Windows 10 technical preview is expected to be available for select Windows Phone 8.1 handsets later this month. Since Microsoft still has a lot of work to do before completing the stable, final version of Windows 10 (this should be released in the summer), a rollback option will be more than welcome.
Besides bringing a more unified look on all devices, Windows 10 for phones will feature a new Notifications system, Spartan browser (which is designed from scratch to eventually replace Internet Explorer), new Settings menu, unified apps, enhanced personalization capabilities, and much more.
source: Twitter via Talking Lumia, Windows Central