Previously I had an Intel based PC with ASUS P5G41-MLX which used BIOS, so my HDD partition was MBR obviously. Recently I have upgraded my PC with UEFI based Motherboard which is MSI 970 GAMING motherboard. But I'm not sure about converting my HDD to GPT for a future OS upgrade or a clean OS installation, as I just plugged my OLD HDD with Windows OS with my new Motherboard. So far there is no issue. As I told earlier that my new Motherboard is UEFI based Motherboard but surprisingly my system information is showing that I'm on LEGACY BIOS mode. Does it mean that I'm running OLD BIOS instead of UEFI?
Check the image below
If you want to use Secure Boot, you need to install Windows 8.1 64-bit in UEFI boot mode. Secure Boot only works in UEFI boot mode, when you enable Secure Boot in your PC's UEFI firmware (BIOS) settings. Secure Boot protect users from rootkits and other low-level malware attacks by blocking unauthorized (non-signed) executables and drivers from being loaded during the boot process.
Here is info about UEFI Boot Mode (installing using the GPT partition style) and Legacy BIOS Boot Mode (installing using the MBR partition style): link
Also if you want to use the Fast Boot hybrid sleep mode, you'll need UEFI. A clean install with windows 8 and above isn't such a pain as it once was as long as you sync your settings w/OneDrive and if you have your libraries on a second internal drive.