Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is the first image in a sequence of a first embodiment of a display screen or portion thereof with transitional graphical user interface showing our new design; FIG. 2 is the second image thereof; FIG. 3 is the third image thereof; FIG. 4 is the fourth image thereof; FIG. 5 is an enlarged view of a portion defined by line 5 - 5 shown in FIG. 1 ; FIG. 6 is an enlarged view of a portion defined by line 6 - 6 shown in FIG. 2 ; FIG. 7 is an enlarged view of a portion defined by line 7 - 7 shown in FIG. 3 ; FIG. 8 is an enlarged view of a portion defined by line 8 - 8 shown in FIG. 4 ; FIG. 9 is the first image in a sequence of a second embodiment of a display screen or portion thereof with transitional graphical user interface showing our new design; FIG. 10 is the second image thereof; FIG. 11 is the third image thereof; FIG. 12 is the fourth image thereof; FIG. 13 is an enlarged view of a portion defined by line 13 - 13 shown in FIG. 9 ; FIG. 14 is an enlarged view of a portion defined by line 14 - 14 shown in FIG. 10 ; FIG. 15 is an enlarged view of a portion defined by line 15 - 15 shown in FIG. 11 ; and, FIG. 16 is an enlarged view of a portion defined by line 16 - 16 shown in FIG. 12 . The outermost perimeter broken line in each drawing illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in the figures depict portions of the transitional graphical user interface that form no part of the claimed design. The dot-dot-dash broken lines encircling portions of the claimed design that are illustrated in enlargements form no part of the claimed design. The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 - 4 and FIGS. 9 - 12 , respectively. The process or period in which one image transitions to another forms no part of the claimed design. The grayscale tones shown in the drawings represent a contrast in appearance.