It's always a little strange to go back to a previous installment of a game series. Things change. But I don't believe it will be disappointing. I think it's enjoyable and nostalgic to replay/revisit a game that's not as stellar as it used to be.
I think I will still enjoy playing them both, although I’ll probably play NL a lot less as I’ll be more obsessed with NH, I’ll probably still be playing NL just because I have projects I want to get done on there and I’ll want to get them done before I take a break from the game.
Not really. Just because a new game release for the series doesn't mean you throw the older ones in the bin. I'll still definitely play it even though it seems like New Horizons addressed most of the issues in the previous games. Each one has their own charm in a way.
It always makes me appreciate the things many may take for granted, either events or tasks that were around in the past but no longer around for newer games (i.e. Animal Crossing Gamecube - Lighting the lighthouse [which I still really want to try someday] and the fall sports fair) or tasks that were made easier in the newer releases (like catching those dang bees ).
With each new installment there are always things they do better and improve on so going back to older versions can be more jarring. That said, it doesn't mean you suddenly hate the older AC games.
The game's not even out yet. You can like an older game in a series better than newer stuff even though some things are improved in newer releases. Gamecube still continues to be my favorite in the series and probably will continue to be since the franchise has taken a different direction than what it started as.