I was using grub to dual boot my Lenovo Y410P with Windows 8 and Ubuntu 14.04.
Recently I finally decided to upgrade my Windows 8 to Windows 8.1 considering it should be pretty stable by now. However, to my horror(or rather not surprisingly), my default bootloader has been changed to Windows Bootloader which means I could not access my Ubuntu 14.04.
Usually this takes a while to fix, involving repairing grub/BCD and tend to be tedious. However, I managed to found a much simpler and direct solution to this.
With the new UEFI specification, we can actually choose the default boot manager instead of replacing one with another. All it takes is to go to UEFI Firmware settings and change the order of boot managers. Which means grub and Windows boot manager can co-exist somewhere in the hard disk and we can choose which one to use in the BIOS UEFI settings.
Accessing UEFI settings in Windows 8.1 is a bit different from what happens in Windows 8, you can follow these links:
http://www.thewindowsclub.com/access-uefi-firmware-settings-windows-8-1
http://acer-au.custhelp.com/app/answers/detail/a_id/32048/~/accessing-uefi-in-windows-8.1
The menu is something like this:
UEFI Settings
One you are in the UEFI boot priority settings, you can simply change the position of Ubuntu to the top so that grub will load when the PC is turned on:
UEFI Boot order priority setting
(All photos taken by me)