Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is a second image thereof; and,

FIG. 3 is a third image thereof.

The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 - 3 . The process or period in which an image transitions to another forms no part of the claimed design. The dot-dash broken lines showing the display screen form no part of the claimed design. The shaded half-tone overlay within the dot-dash lines illustrate the graphical user interface that forms no part of the claimed design. The dash-dash lines define the boundary of the graphical user interface being claimed. The figures are shown in grayscale gradient that represents a contrast in appearance, the specific grayscale gradient shown forms no part of the claimed design.