Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a first image of a display screen or portion thereof with transitional graphical user interface according to the claimed design; and, FIG. 2 is a front view of a second image according to the claimed design. The outermost long-dashed broken lines show an electronic device that forms no part of the claimed design. The shorter-dashed broken line rectangle shows a display screen and forms no part of the claimed design. Within the greyscale buttons, white depicts an opaque light area and shading indicates a contrast in appearance. All other portions of the design shown in white form no part of the claimed design. The appearance of the transitional image transitions between the images shown in FIGS. 1 - 2 . The process or period in which one image transitions to another forms no part of the claimed design.