One, windows 8 and 8.1, OEM or retail, don't care how many hardware changes you make, they will still work and are not tied to a single motherboard like Windows 7, however, after making a major change you WILL have to reactivate online or by phone and verify you only have the OS installed on one system.
Two, if you have files that need to be backed up, then upload them to a free file hosting service or you will need to purchase an additional drive for data storage and backup purposes, which you should have anyhow. Not having important files backed up to at least one and preferably two (ie. a secondary drive or two and possibly optical disks like DVD or Blu Ray) is asking to lose all your important files.
Hard drive failure isn't a question of if, but when. And once it happens, it's too late. It's gone. Even data recovery services, which are VERY expensive, can't always get your data back depending on the type of failure. Get another drive to back your data up to. If it's not worth the 50 bucks a 1TB drive costs, then it isn't that important after all.
If you have plans to upgrade to Windows 10 though, make sure to do the motherboard change BEFORE upgrading, otherwise you'll have to go through the hassle of dealing with a phone activation after changing the motherboard.