Can I convert my Win8.1 disk to GPT?

First some background:
GPT stands for GUID Partition Table. It’s a new standard that’s gradually replacing MBR. It’s associated with UEFI — UEFI replaces the clunky old BIOS with something more modern, and GPT replaces the clunky old MBR partitioning system with something more modern. It’s called GUID Partition Table because every partition on your drive has a “globally unique identifier,” or GUID — a random string so long that every GPT partition on earth likely has its own unique identifier.

This system doesn’t have MBR’s limits. Drives can be much, much larger and size limits will depend on the operating system and its file systems. GPT allows for a nearly unlimited amount of partitions, and the limit here will be your operating system — Windows allows up to 128 partitions on a GPT drive, and you don’t have to create an extended partition.

On an MBR disk, the partitioning and boot data is stored in one place. If this data is overwritten or corrupted, you’re in trouble. In contrast, GPT stores multiple copies of this data across the disk, so it’s much more robust and can recover if the data is corrupted. GPT also stores cyclic redundancy check (CRC) values to check that its data is intact — if the data is corrupted, GPT can notice the problem and attempt to recover the damaged data from another location on the disk. MBR had no way of knowing if its data was corrupted — you’d only see there was a problem when the boot process failed or your drive’s partitions vanished.

MBR works with disks up to 2 TB in size, but it can’t handle disks with more than 2 TB of space. MBR also only supports up to four primary partitions — if you want more, you have to make one of your primary partitions an “extended partition” and create logical partitions inside it. This is a silly little hack and shouldn’t be necessary. Windows can only boot from GPT on UEFI-based computers running 64-bit versions of Windows 8.1, 8, 7, Vista, and corresponding server versions. All versions of Windows 8.1, 8, 7, and Vista can read GPT drives and use them for data — they just can’t boot from them. Ref: http://www.howtogeek.com/193669/whats-the-difference-between-gpt-and-mbr-when-partitioning-a-drive/

To answer your question:
Yes it is possible to convert the disk to GPT without losing any data Here is a tutorial http://www.disk-partition.com/gpt-mbr/convert-mbr-to-gpt-without-data-loss.html