Display Screen or Portion Thereof with Transitional Graphical User Interface

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

FIG. 2 is a second image thereof.

The appearance of the transition is sequential from FIG. 1 to FIG. 2 of the claimed transitional graphical user interface.

The process or period in which an image transitions to another image forms no part of the claimed design.

The dash-dash broken lines shown in the figures are for illustrative purposes only and form no part of the claimed design.

The dash-dot broken lines show portions of the display screen that form no part of the claimed design.