Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a display screen or portion thereof with transitional graphical user interface showing our new design;

FIG. 2 is a front view of a second image thereof; and,

FIG. 3 is a front view of a third image thereof.

The outermost broken line in the figures depicts a display screen or portion thereof that forms no part of the claimed design.

The remaining broken lines in the figures depict portions of the graphical user interface which form no part of the claimed design.

The appearance of the graphical user interface sequentially transitions between the images shown in FIGS. 1 through 3 . The process or period in which one image transitions to another forms no part of the claimed design.