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 our design; and,

FIG. 2 is a front view showing a second image thereof.

The appearance of the image transitions sequentially between the image shown in FIG. 1 to FIG. 2 . The process or period in which one image transitions to another forms no part of the claimed design.

The shading depicts a contrast in appearance.

The outermost long-dashed broken lines illustrate an electronic device, which constitutes environment of the design and forms no part of the claim. The medium-length-dashed broken line illustrates a display screen and the boundary of the design and forms no part of the claim. The remaining short-dashed broken lines illustrate portions of the graphical user interface that form no part of the claimed design.