After the upgrade make a clean install. Download and use the newest drivers for Windows 10 and everything will work just normal. Next month Windows 10 will get a major update with a lot of refinements, fixes and features. One of them is that you will no longer need to upgrade through your 7/8 and then make clean install. You will be able to install W10 straight away and use your Windows 7/8 license key straight away to activate Windows 10. So the process will be simpler faster and safer. FFXIV runs without problems on 10. Older systems may get a slight performance improvement as well. Just be sure to use the newest drivers.