For most of the time, I'd say no, based on the concept of ex post facto. In other words, if a sentence for a crime would be more severe today than it was at the time the crime was committed, I don't think you could justifiably lengthen that sentence. (In fact, I'm not sure if lengthening a sentence is allowed under any circumstances; there could be some constitutional issue surrounding that.) However, in certain circumstances, I could see where a sentence for a crime could be substantially lessened, particularly involving a civil rights or discrimination issue.