Display Panel or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a first embodiment of a display panel or portion thereof with transitional graphical user interface, showing a first image in a sequence showing my new design; FIG. 2 is a second image thereof; FIG. 3 is a front view of a second embodiment of a display panel or portion thereof with transitional graphical user interface, showing a first image in a sequence showing my new design; and, FIG. 4 is a second image thereof. The outermost dash-dot broken lines in the drawings illustrate the display panel or portion thereof and form no part of the claimed design. The other broken lines in the drawings illustrate portions of the graphical user interface that form no part of the claimed design. The appearance of the graphical user interface changes between the images shown in FIGS. 1 - 2 , and FIGS. 3 - 4 , respectively. The process or period in which one image transitions to another forms no part of the claimed design.