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 a first image in a sequence of my new design; and,

FIG. 2 is a front view showing a second image in the sequence.

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 to another image forms no part of the claimed design.

The outmost broken lines illustrate the display screen and form no part of the claimed design. The remaining broken lines, illustrating portions of the graphical user interface, form no part of the claimed design.