Display screen or portion thereof with transitional graphical user interface

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

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof; and,

FIG. 4 is a fourth image thereof.

The outer broken lines show the display screen or portion thereof and form no part of the claimed design. The broken lines showing a person's hand is for environmental purposes only and form no part of the claimed design. The remaining broken lines show elements of the graphical user interface that form no part of the claimed design. The solid greyscale circular element forms no part of the claimed design. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-4 . The process or period in which one image transitions to another forms no part of the claimed design.