Display screen or portion thereof with transitional graphical user interface

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

FIG. 2 is the second image thereof;

FIG. 3 is the third image thereof;

FIG. 4 is the fourth image thereof; and,

FIG. 5 is the fifth image thereof.

The dashed broken lines around the figures show a display screen or portion thereof and form no part of the claimed design.

The dashed broken lines within the figures show unclaimed portions of the graphical user interface and form no part of the claimed design.

The subject matter in this patent includes a process or period in which an image changes into another image. This process or period forms no part of the claimed design.