The ornamental design for a blank for changeable alpha numeric characters, as shown and described.
FIG. 1 is a front perspective view of a blank for changeable alpha numeric characters showing my new design with the broken line showing for illustrative purposes only and forming no part of the claimed design;
FIG. 2 is a rear plan view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and
FIG. 5 is an enlarged partial front bottom left corner thereof.