Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a front view of a display screen or portion thereof with transitional graphical user interface showing my new design in a first state;

FIG. 2 is a front view thereof in a second state;

FIG. 3 is a front view thereof in a third state; and,

FIG. 4 is a front view thereof in a fourth state.

The broken lines in the figures depict environmental subject matter and form no part of the claimed design. The outermost broken lines in the figures show a display screen or portion thereof and form no part of the claimed design. The appearance of the animated user interface sequentially transitions between the images shown in FIGS. 1-4 . The process or period in which one image transitions to another forms no part of the claimed design.