Display screen or portion thereof with transitional graphical user interface

FIG. 1 is the first image in a sequence of a display screen or portion thereof with transitional graphical user interface showing our new design;

FIG. 2 is the second image thereof;

FIG. 3 is the third image thereof;

FIG. 4 is the fourth image thereof;

FIG. 5 is the fifth image thereof;

FIG. 6 is the sixth image thereof; and,

FIG. 7 is the seventh image thereof.

The broken lines show a display screen or portion thereof and portions of the graphical user interface which form no part of the claimed design.

The subject matter in this patent includes a process or period in which an image changes into another image. This process or period forms no part of the claimed design.