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 dot-dash broken lines in the drawings show a display screen or portion thereof and form no part of the claimed design. The remaining broken lines in the drawings, including evenly spaced broken lines and all broken line text, show portions of the transitional graphical user interface, or an electronic device having a display screen or portion thereof, and form no 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.