I'm mention in the patch notes if saves ever break. I try to not break saves when possible. And, at most, I expect it to happen maybe once before the end of development. But hopefully just that once.
For downloading, yes. Any release with a v#.#.0 involves new content+assets, and will have assets you need to download to continue the game. It's a mess logistically to try and make patches in these cases since its so situation dependent, so the game files need to be downloaded. You can then move your Save folder over to the new files
For romance, it depends on the character (will be open that Harper specifically has no romance content planned. That connection is more familial like)