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; and,

FIG. 2 is the second image thereof.

The outer perimeter shown in broken lines in the figures depicts a display screen or portion thereof and forms no part of the claimed design.

The remaining broken lines in the figures depict portions of the transitional graphical user interface which form no part of the claimed design.

The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 and 2 . The process or period in which one image transitions to another forms no part of the claimed design.