I claim the ornamental design for a display screen or portion thereof with transitional graphical user interface, as shown and described.
FIG. 1 is a first image in a sequence of a display screen or portion thereof with transitional graphical user interface showing my new design; and,
FIG. 2 is a second 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 show 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.