Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a front view showing a first image in a sequence according to the claimed design;

FIG. 2 is a front view of a second image in the sequence thereof;

FIG. 3 is a front view of a third image in the sequence thereof; and,

FIG. 4 is a front view of a fourth image in the sequence thereof.

The appearance of the transition is sequential from FIG. 1 through 4 . The process or period in which an image transitions to another image forms no part of the claimed design.

The longer-incremented broken lines illustrate an electronic device, and the intermediate-length broken lines illustrate a display screen or portion thereof. The remaining broken lines illustrate portions of the graphical user interface. None of the broken lines form part of the claimed design.