I claim the ornamental design for a display screen or portion thereof with transitional graphical user interface, as shown and described.
FIG. 1 is the first image in a sequence of a display screen or portion thereof with transitional graphical user interface showing my new design;
FIG. 2 is the second image thereof; and,
FIG. 3 is the third image thereof.
The outermost broken lines showing the display screen or portion thereof are for environmental purposes only and form 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 graphical user interface sequentially transitions between the images shown in the drawing figures. The process or period in which one image transitions to another forms no part of the claimed design.