Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a first image in a sequence for a display screen or portion thereof with transitional graphical user interface;

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

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

The thicker vertical dash-dash broken lines in FIGS. 1 and 3 within the graphical user interface form part of the claimed design.

The thinner dash-dot broken lines in the drawings illustrate a display screen or portion thereof and form no part of the claimed design. The remaining thinner dash-dash lines illustrate portions of the transitional graphical user interface that 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 image forms no part of the claimed design.