Letters can be repunched to extend the life of a blocked die. Equally, dies can be polished to remove old detail and a new legend/design entered. Obviously this depends on the depth of the relief, and is mostly restricted to hammered coins. From the advent of milled coins up to the 19th century, it was quite common to fill the old die and punch new letters over the top of the filled section. Depending on how effectively the die was filled, you may or may not see traces of the underlying detail.