Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is the 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 the second image thereof.

The outermost broken line rectangle in the drawings illustrate a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in the drawings illustrate portions of the graphical user interface and form no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 2 . The process or period in which one image transitions into another image forms no part of the claimed design.