Microsoft began rolling out the Windows 10 Mobile over-the-air update to selected Windows Phone 8.1 devices in March. However, if those folks that do receive the update decide that Windows 10 Mobile is not for them, they will continue to have the option to downgrade back to Windows Phone 8.1.
@heynerandresgom Devices that originally launched on WP8 will keep WP8 recovery images in the tool.— Jason (@NorthFaceHiker) April 4, 2016
This is good news for people who might encounters bugs and other issues with Windows 10 Mobile, or if they just like Windows Phone 8.1 better. Jason also confirmed that there are currently no plans to offer Windows 10 Mobile images via the Windows Phone Recovery Tool.