Display Screen or Portion Thereof with Transitional Graphical User Interface

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. FIG. 1 is a perspective view of a display screen or portion thereof with transitional graphical user interface showing a first image of a first embodiment of my new design; FIG. 2 is a front view of the first image; FIG. 3 is a second image thereof; FIG. 4 is a third image thereof; FIG. 5 is a fourth image thereof; FIG. 6 is a fifth image thereof; FIG. 7 is a sixth image thereof; FIG. 8 is a seventh image thereof; FIG. 9 is an eighth image thereof; FIG. 10 is a ninth image thereof; FIG. 11 is a tenth image thereof; FIG. 12 is an eleventh image thereof; FIG. 13 is a twelfth image thereof; FIG. 14 is a perspective view of a display screen or portion thereof with a transitional graphical user interface showing a first image of a second embodiment of my new design; FIG. 15 is a front view of the first image; FIG. 16 is a second image thereof; FIG. 17 is a third image thereof; FIG. 18 is a fourth image thereof; FIG. 19 is a fifth image thereof; FIG. 20 is a sixth image thereof; FIG. 21 is a seventh image thereof; FIG. 22 is an eighth image thereof; FIG. 23 is a ninth image thereof; FIG. 24 is a tenth image thereof; FIG. 25 is an eleventh image thereof; and, FIG. 26 is a twelfth image thereof. The broken lines in the drawings show the electronic device and portions of the display screen or portion thereof with transitional graphical user interface, and form no part of the claimed design. The appearance of the transitional images sequentially transitions between the views of FIGS. 1 - 13 and FIGS. 14 - 26 . The process or period in which an image transitions to another forms no part of the claimed design.