The ornamental design for a convertible faceshield, as shown and described.
FIG. 1 is a left perspective view of the convertible face shield installed on a cap where the tinted portion covers the wearer's eyes;
FIG. 2 is another left perspective view showing the convertible face shield installed on a cap where the clear portion covers the wearer's eyes thereof;
FIG. 3 is a front plan view, the back plan view being a mirror image thereof; and,
FIG. 4 is a left side view, the right being a mirror image thereof.
The broken lines showing of a cap and human face are shown for environmental purposes and form no part of the claimed design.