Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 a front view of a display screen or portion thereof with transitional graphical user interface showing a first image of the claimed design; FIG. 2 is a second image thereof; and, FIG. 3 is a third image thereof. The outermost evenly-spaced broken lines show an electronic device, which is the environment of the claim and forms no part of the claimed design. The dot-dashed broken lines adjacent to the outermost evenly-spaced broken lines show a display screen or portion thereof and form no part of the claimed design. The remaining broken lines in the drawings show portions of the transitional graphical user interface and form no part of the claimed design. The appearance of the transitional image sequentially transitions between the views shown in FIGS. 1 - 3 . The process or period in which an image transitions to another forms no part of the claimed design.