I tried to figure that out, and did read what I figured were the relevant articles, but on this the constitution seems vague. I think that a case could be made that with the expiration of terms there is no congress, period. The Senators who have time left on their term are automatically going to be in the next congress, but until that congress is seated they really can't do much of anything and that congress can't be seated until both houses are represented. That was my take on it and I'd argue it, but if someone finds something applicable and can make a better case I'm not gonna fight it.
For the record, there are 33 democrats, 30 republicans, and 2 independents carrying over into the next senate. Currently. At this point it is realistic to note that some of them may die between now and January.