Actually, I only needed to touch, not reset the screen. This is better.

src/font.c

@@ -563,7 +563,6 @@ change_font(int init, const char *fontname)
   /* If the sizes haven't changed, we don't have to update the hints */
   if (fw == TermWin.fwidth && fh == TermWin.fheight) {
      /* but we _do_ need to redraw to show the new font */
-     scr_reset();
      scr_touch();
      return;
   }