Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a front view showing a first image in a sequence of 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;

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

FIG. 5 is a front view of a fifth image in the sequence thereof.

The appearance of the transition is sequential from FIG. 1 through FIG. 5 . The process or period in which an image transitions to another image forms no part of the claimed design.

The dot-dash lines indicate bounds of the claimed design and form no part thereof.

The longer-incremented dash-dash broken lines illustrate an electronic device, and the dash-dash broken line rectangle illustrates a display screen or portion thereof. The remaining dash-dash broken lines illustrate portions of the graphical user interface. None of the broken lines form part of the claimed design, and none of the white portions illustrated in the drawings form part of the claimed design.