The sources seem to indicate that Nintendo is looking into an upgrade for the current screen. The sources hint that the upgrade would lead to slightly better image quality, brightness, and better energy efficiency. At present, the Switch uses a lower-end LCD for their display. And while it won't make the jump to something like an LED display or even OLED, moving to a higher-end LCD could help a bit.
And what else can we expect? Well uh... that's a really good question, isn't it? It seems as though an updated panel is all we really have mentioned by any of these sources right now. Nintendo is still apparently "debating what new hardware and software features to include in the upgrade." I would maybe assume we would see a slightly faster CPU/GPU? Just don't expect to see any massive leap in terms of power.
The Wall Street Journal says that Nintendo is considering this move to give another boost to Switch sales, which have started to slow up a bit from its very strong launch in 2017. If this hardware revision does happen, you may see it as soon as Summer 2019.
