Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a front view of a black and white version of a first image of a first sequence of a display screen or portion thereof with transitional graphical user interface according to the claimed design;

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

FIG. 3 is a front view of a third image of the first sequence;

FIG. 4 is a front view of a first image of a second sequence according to the claimed design;

FIG. 5 is a front view of a second image of the second sequence; and,

FIG. 6 is a front view of a third image of the second sequence.

The longer-length dashed broken lines show an electronic device and form no part of the claimed design. The shorter-length dashed broken lines show the perimeter of a display screen and various elements within the graphical user interface, and form no part of the claimed design. The dot-dash broken line shows a portion of the graphical user interface, and forms no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 3 in a first embodiment and FIGS. 4 - 6 in a second embodiment, respectively. The process or period in which one image transitions to another forms no part of the claimed design.