While it may currently seem unusual for OnePlus to introduce such a big change for an incremental hardware upgrade, the news comes from the reliable Ishan Agarwal. The OnePlus 7T will apparently feature a larger display with a small notch than its predecessor that will also operate at up to 90 Hz. The next OnePlus flagship seems a curious upgrade from the OnePlus 7 for other reasons too according to leaked specs.
https://www.notebookcheck.net/Massive-upgrade-Regular-OnePlus-7T-to-feature-a-larger-and-90-Hz-display.433544.0.html