It could be an "intra-regimental company transfer," but there's no special order or muster roll notation affirming that. It's also possible that there was a field consolidation of the two companies toward the end of the siege, or he might have been temporarily detailed to Co. E, or there might simply have been a clerical error on the tabulation of prisoners (which happened a LOT); the record does not shed any light on the subject. All we can do is speculate why he was carried on the rolls of Co. D, but was listed in Co. E at the surrender.