I've got a late 2015 5k iMac and I made the unfortunate mistake of not speccing a big enough drive when I purchased it. I don't have a problem with the spec of the machine although it's probably going to get an upgrade in the next year or so. Would you bite the bullet and upgrade the whole machine now or upgrade the drive instead? Is it actually quite difficult? I have seen the instructions but can't really gauge how difficult it will actually be. Thoughts?