Display screen or portion thereof with a transitional graphical user interface

1 FIG. is a view of a display screen or portion thereof with a transitional graphical user interface, showing the new design; 2 FIG. is another view thereof after a first transition; and, 3 FIG. is another view thereof after a second transition. In the drawings, the dashed line perimeter illustrates aspects of a display screen or portion thereof and forms no part of the claimed design. The broken lines illustrate aspects of the transitional graphical user interface of the display screen or portion thereof and form no part of the claimed design. 1 3 FIGS.- The appearance of the transitional image sequentially transitions between the images shown in. The process or period in which one image transitions to another image forms no part of the claimed design.