According to Verizon’s website, the Galaxy S8 should receive software version G950USQU1AQD9, while the Galaxy S8+ receives G955USQU1AQD9. For the T-Mobile models, as shown in the screenshot below, it will be the same exact thing.
As for what’s changing, the Galaxy S8+ for T-Mobile lists improvements for Face Recognition, tweaks for the Camera user interface, plus the April security update. For Verizon, the carrier details optimizations for the Infinity display, the added ability to press the volume up button to answer calls, and tweaks for notification badges.
Owners on T-Mo and Verizon, go get these updates.