When you installed your current OS you booted the normal installer. To install in UEFI you would need to boot the UEFI Windows setup instance.
There is not a way worth your time to convert your current disk. You would be better off re-install 8.1 fresh from the ISO.
1) When you turn on the computer boot the UEFI Windows 8.1 USB or DVD
2) When the installer boots hit SHIFT+F10 to launch a command prompt.
3) Then run "diskpart", then "list disk", then "select disk X".
4) List disk again "list disk" you will notice the asterisk on the left confirming the disk you just selected. The asterisk on the the right for GPT will be missing because you are MBR formated.
5) clean the disk with the "clean" command which will clean the select disk
6) "exit" command
7) "exit" command again
Now go through the windows setup and when you do custom install you will see your un-partitioned disk. Create your Windows partition and if your in UEFI mode it will create 3 smaller partitions in the front of the windows partition. If you have less then four paritions something is not right.
If all went well you can hit SHIFT+F10 again and run diskpart. If you run the "list disk" command you will see the disk is now GPT enabled with the asterisks confirming so. Now just hit next to start the install.
Hope this helps
P.S. Is is OK if you computer can not do secureboot, this enables extra security and also lets users boot UEFI video BIOS for fastboot into windows.