Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a first image in a sequence for a display screen or portion thereof with transitional graphical user interface showing our new design;

FIG. 2 is a second image in a sequence thereof; and,

FIG. 3 is a third image in a sequence thereof.

The outermost broken line illustrates a display screen or portion thereof and forms no part of the claimed design.

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

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