I have a crown that the tooth under the crown has darkened. This is a front tooth and in this case the gums are receding somewhat. Can a crown have a venere placed on it vice having the crown replaced when it appears to have darkened? There has been no pain resulting from this and as yet no discomfort at all. The crown is spmething over 40 years old. If the crown must be replaced can you please describe the procedure and possible complications?