Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of an embodiment of a display screen or portion thereof with a transitional user interface showing a first image; and, FIG. 2 is a front view of a second image thereof. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 2 . The process or period in which one image transitions to another image forms no part of the claimed design. The rectangular broken-line perimeter illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines showing portions of the graphical user interface form no part of the claimed design.