As near as I can tell it looks like mostly exterior & interior appearance changes.
Here is a fairly long video that goes into much detail of the interior and goes for a short test drive.
https://insideevs.com/2019-kia-niro-ev-worth-tesla-model-3-money/ Note the smoking tires LOL! The info I have seen implies but doesn't explicitly state if all the changes will be in the 2020 PHEV or not.
Sounds like the interior gets more soft touch surfaces and is glitzed up with color. I kind of like the blue color in the seat perforations, neutral on the rest of the interior changes.
Lord willing I hope to buy a Niro PHEV EX Premium in the next 6 months. Unfortunately I was not ready to pull the trigger earlier when they had 0% financing and a $1K rebate :| So I'm waiting for that to return :x
I kinda wish I didn't have the additional tempting "new & improved" factor! I'll have to see it in the flesh to determine which style I like best and then see what sort of end of model year discounts may be available for the last of the 2019s.