Re: Interest Check - Buffy the Cyberpunk Vampire Slayer (??)
There's a lot of ways you can go with things, which I agree will modify how things feel.
A lot of the combat capabilities could translate 1:1. For a Fighter, the Defense Fighting Style works just as well whether the combatant is wearing plate or some sort of polymer laced long coat. The archery style could simply apply to firearms as well as other ranged weapons. You could allow the Great Weapon Fighting ability to apply to longarms.
A lot of "magic" could just as easily be technology. Minor Illusion just becomes holographic projectors, Cure Wounds a trauma slap patch, etc.
I think it would be a lot easier on the GM to modify things on a case by case basis depending on player concept then try to convert the entirety of player options wholesale, or reinvent the wheel from the ground up.
However I'd be interested in giving it a shot whether you decide on a simpler type of update or something much more ambitious.