Convert HDD Partition Type Without Data Loss

So I have a 3TB HDD and since I have got it, it has always shown 2TB. I assumed I got the wrong hdd, however looking in disc manager, I do not, there is 746.52gb unallocated, I cannot allocate it, so I looked it up, I have to make the drive GPT instead of MBR, but I already have almost 2TB of stuff on there that cannot lose access to, and I need to be able to allocate that other 1tb I cant access. Is there a way to convert the HDD from MBR to GPT without data loss? I might have to find multiple hard drives from old laptops to temporarily store data but I really do not want to do that, any ideas?