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 according to the design;

FIG. 2 is the second image thereof; and,

FIG. 3 is the third image thereof.

The outermost dash-dot-dash broken line rectangle showing a display screen forms no part of the claimed design.

The appearance of the image 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.