...this calls forward the question: should he be second billed at all? I mean, early in the show, the second most important role definitely seemed to be either Cat or Alex. For a bit, it was Winn and Alex, then Mon-El and Alex. These days, it seems to me to either be Dreamer or Alex. The consistent factor is Alex, and I think that actress should be second billed. But I'd understand if the actor playing James would feel slighted by that, since he was brought onto the show with second billing.
It's not about feelings, it's about contracts. Credit placement is related to how much money you make. Moving someone back in the credits means cutting their salary -- unless you stick them at the end with a "With/And Actor Name as Role," which is seen as prestige billing. (This is why Michael Shanks got such a last-place "as Daniel Jackson" credit when he returned to Stargate SG-1 after a season away. The actors after him in the credits had been bumped up in his absence, and it wouldn't have been fair to demote them when he came back.)
By the same token, this is why Brandon Routh is billed ahead of Caity Lotz on Legends of Tomorrow, and why Victor Garber was billed first until his departure, despite having an increasingly small role. Credits are strictly about contracts and salaries, regardless of what happens with an actor's relative story prominence.
The only times I've ever seen a new cast member inserted into the billing above an older cast member have been when the new cast member was installed as the lead, like Ben Browder on Stargate SG-1 or Bruce Boxleitner on Babylon 5.
But then, I've also heard a rumor (the reliability of which, I have no idea) that the show will become Superman next season.
First I've heard of it, and it sounds like the usual Internet nonsense.