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 the first image in a sequence of our new design; and,

FIG. 2 is a front view of the second image thereof.

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

The long and short dashed broken lines in the drawings illustrate the display screen or portions thereof and form no part of the claimed design. The remainder of the graphical user interface shown in broken lines in the drawings illustrates environment and forms no part of the claimed design.