How to convert bootable MBR system disk to a UEFI bootable disk without losing your data in Windows 8/8.1
Repair UEFI system boot partition (only perform steps 1, 5.ii, 6-7).
You need to have a working Windows installation on a bootable MBR disk which has two partitions - a 100 MB system MBR partition (id=27) and a primary Windows partition with intact C:\Windows.
In order to boot Windows in UEFI mode, you will have to convert your disk to GUID Partition Table (GPT) format, create (or re-create) the necessary system partitions, copy EFI Windows Boot Manager code to the System partition and create BCD store. Your Windows partition will be left intact and all your programs, files and settings will remain.
0. Create a System Image Backup
Control Panel, File History, System Image Backup
Select a hard disk to store the backup. It should be a basic disk (NOT a dynamic disk) and should have enough free space to hold your entire C: partition.
The backup should only be required if something goes very wrong with partition resize in step 3 - for example, if you accidentally wipe your main partition, or if a power surge interferes with non-destructive resize operation, in which case you should boot Windows Recovery Environment (WinRE) and restore your partitions.
Note that if you decide to wipe the disk and run a clean install using Windows Setup media, this backup will NOT allow you to restore your files afterwards, since the size of the new partition will not match. You can only restore the full system partition as it was at the time of the backup.