Display screen or portion thereof with transitional graphical user interface

The patent file contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Office upon request and payment of the necessary fee. 1 FIG. is a front view of the first image in a sequence for a display screen or portion thereof with transitional graphical user interface showing our new design; and, 2 FIG. is the second image thereof. The outer perimeter dashed broken lines in the drawings illustrate a display screen or portion thereof, and form no part of the claimed design. The other broken lines in the figures form part of the claimed design. 1 2 FIGS.- The appearance of the transitional image sequentially transitions between the images shown in. The process of period in which one image transitions to another image forms no part of the claimed design.