Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a first image in a sequence of a display screen or portion thereof with transitional graphical user interface 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; FIG. 4 is a front view of a fourth image thereof; FIG. 5 is a front view of a fifth image thereof; FIG. 6 is a front view of a sixth image thereof; FIG. 7 is a front view of a seventh image thereof; FIG. 8 is a front view of an eighth image thereof; FIG. 9 is a front view of a ninth image thereof; FIG. 10 is a front view of a tenth image thereof; FIG. 11 is a front view of an eleventh image thereof; FIG. 12 is a front view of a twelfth image thereof; FIG. 13 is a front view of a thirteenth image thereof; FIG. 14 is a front view of a fourteenth image thereof; FIG. 15 is a front view of a fifteenth image thereof; and, FIG. 16 is a front view of a sixteenth image thereof. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 16 . The process or period in which one image transitions to another image forms no part of the claimed design. The outmost broken lines illustrate the display screen and form no part of the claimed design. The white broken line numbers in the figures form part of the claimed design.