Can you reuse a screen protector on a new phone?

Display diagonal is only one variable. The usable front glass, bezel, corner shape, sensors and cutouts all affect protector fit.

Why “same screen size” can mislead

A 6.3-inch display on two generations does not prove the same protector works. The chassis can change slightly, bezels can shift, and cutout-style protectors can conflict with a changed camera or sensor area.

When reuse is most plausible

Reuse is more plausible when the phone maker publishes the same front dimensions and the protector is a simple full-cover design without model-specific cutouts. Even then, confirm the accessory maker's compatibility list.