That would maybe apply if he was in an exchange-supported intra-calendar spread. But I sense that is not what he is asking. Going to have to throw him a @bone on this one. Hehe
If you can execute both orders within a very short time span you would not have the risk of a sudden spike in price (e.g. within a couple of seconds). Neither during nor outside regular trading hours. Executing your trades outside regular trading hours carries the risk of being exposed to wider bid/ask spreads. This will make you lose money on both the position you are closing, as well as on the position you are opening. Most likely is the bid/ask spread smaller during regular trading hours.