I'm currently looking at doing similar upgrades, albeit with a Ryzen 5 1400 and from what I gather 1) 1440p becomes a lot more GPU bound, so you'll probably not lose a ton of performance depending on the titles, and 2) it depends on what fps targets you are aiming for. I would bet at 60hz, you'll probably be pretty OK, esp. if you can OC the 2500k.
I found this in my own searching on the ryzen question and at 1440p, you can see that it's keeping up with other Intel and Ryzen chips (linked below). It definitely is slower at 1080p, but seems to be above 60fps on average. And since it's AM4, I can drop in a chip just about at any time w/o needing a new board. Also, that R5 3600 seems to kick ass so you can't go wrong with an upgrade - definitely got a lot of life out of the 2500k.
It would be be better too upgrade the gpu but it will still be bottlenecked and upgrading the cpu would not make much of a difference so I would suggest saving up for an upgrade for both or if you can’t I would suggest a gpu upgrade (the 5700xt is faster than the 2060 super
