How do I watch replays that I have downloaded but are from previous patch?

Hey. I am sorry but you can't watch replays from the previous patches. Even if you downloaded you can't. But a good feature you can use is the "rec" button while you are watching a replay. With it you can recorde your favourite team fight or solo pentakill. After that you can watch this video over and over again even in different patches. You can access all your recordings in the "Highlights" menu that is placed in the "Profile" menu.

I hope that helped. If you have more questions feel free to ask.

You can't because ... because that's badly implemented mainly.

You cantbecause replays are just a set on inputs that determined how the game was played. Once the next patch hits the inputs are no longer valid because the actual game has changes.