Ok, you have a lot of questions that might be worth going into if you really need to know all of them, but I'll try to get at the heart of your query.
To your overall premise: will my video on my hard drive degrade over time?
The simple answer is NO ... it is digital and will stay the exact same quality as when you put it on there. Analog tapes and the stretching, tearing, heating and cooling etc ... those suffered. Hard drives don't need to worry about that kind of degradation to the data over time.
That said, there are some things that will affect hard drive longevity, much like a tape. How hot will the hard drive get? How many spin up/spin downs do you have it do over the life of the drive?
Mechanical hard drives are still ... well ... mechanical; moving parts can fail. The heads can cause errors on the disc, bad things can happen to data if there's a virus, or you allow some family member access to delete your data (there's recovery methods but that's another topic) and all these things can cause a video file to be rendered unplayable.
SATA is backwards compatible.
Is it worth transferring all your data to a new hard drive? Maybe ...
I had a bunch of family photos and video on an old IBM Deskstar drive ... after 3 years I moved all the data onto another drive (a copy) just in case there was a hard drive failure, and like clockwork the Deskstar drive died a few months after warranty
... but I had seen these kinds of issues online so I was ready for it. At year one would it have been worth it for me to move the files ?? no, but how was I to know that?
The bottom line, if these videos are important you may want them on two hard drives, just as a precautionary back up. Hard drive failure happens ... unlike those old analog tapes you can't just open the cassette up and do a bit of cutting and taping to get a hard drive functional again. Hard drives are very inexpensive now. I have highly recommended to all my friends that they just buy a hard drive to duplicate their family photos on.
You wanted to know if there was anything software related that can affect the quality of the media ... I guess I'm not sure how to answer that part of your question.