Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a first image in a sequence of a display screen or portion thereof with transitional graphical user interface; FIG. 2 is a front view of a second image in the sequence thereof; FIG. 3 is a front view of a third image in the sequence thereof; and, FIG. 4 is a front view of a fourth image in the sequence thereof. The appearance of the design transitions sequentially between FIGS. 1 - 4 . The process or period in which one image transitions to another forms no part of the claimed design. The outer broken line circle showing an electronic device, the inner broken line circle showing the display screen or portion thereof, and all other broken lines showing portions of the graphical user interface form no part of the claimed design.