D&D 5E Surprise with a fight in progress is possible?

[jrowland]

It makes sense if the change would tempt the GM to say, "This is now a new combat for all involved." If the new addition really changes the situation so drastically that everyone would stop and reconsider what they're doing, you might well call for surprise.

I've done this. (HotDQ, in chapter 3) when a thunderwave spell alerts nearby rooms. A few rounds later reinforcements arrive. In this case, it was Big Bad Blue (BBB), and to reinforce the significance of the event, I made EVERYONE re-roll initiative at the end of the current round, giving the new groups a bonus +10. For that 1st (new) round everyone but the new group was considered surprised (including the enemies they were still fighting!) until they acted on their initiative. The rogue got the jump on an 'old' enemy and got a good sneak attack in and BBB drop a PC to zero HP, so not that big of a deal...but the tension really was highlighted as I thought it would and the combat dynamic shifted. The players thought a TPK was imminent and fought like dead men: and won. Good times.