Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is a front view of a second image thereof;

FIG. 3 is a front view of a third image thereof; and,

FIG. 4 is a front view of a fourth image thereof.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 4 . The process or period in which one image transitions to another image forms no part of the claimed design. The rectangular broken-line perimeter illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines shown in the drawings form no part of the claimed design.