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 of the new design; and, FIG. 2 is a second image thereof. The outermost evenly spaced broken lines in the drawings show the electronic device, which is the environment of the design and forms no part of the claimed design. The dot-dash broken lines showing the display screen or portion thereof form no part of the claimed design. The remaining broken lines showing portions of the graphical user interface form no part of the claimed design. The stippling in the drawings shows a contrast in appearance and forms part of the claimed design. The appearance of the transitional image sequentially transitions between the views of FIGS. 1 - 2 . The process or period in which an image transitions to another forms no part of the claimed design.