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 showing our new design; and, FIG. 2 is a front view of a second image in the sequence thereof. The outermost dash-dot broken lines in the drawings illustrate the display screen or portion thereof and form no part of the claimed design. The appearance of the transitional image transitions between the images shown in FIGS. 1 through 2 . The process or period in which one image transitions to another forms no part of the claimed design.