Keep my blank page template in theme after updating wordpress plugin

I have added several webpages using my blank page template within my TwentyFifteen theme. However after updating the WordPress to the latest 4.4, I discovered that all my pages are reverted to the theme instead of blank (without headers, sidebars and background).

Kindly advise how I can keep my pages with the blank page template even after updating? or if there is a shortcut/ tip I can use to convert my webpage to blank theme easily?

It is very time consuming to repeat the process again especially if I have more than 2 pages.