Rewrite the percentages of every message attached to a split campaign in one call, applying the rules of the campaign wizard: an even split is reset to equal shares, and any other split type has to leave part of the audience for the winning message.

The body has to cover every message attached to the campaign. A partial update is a 400, since it would otherwise leave the campaign with a split the wizard would have rejected.

Only split (A/B) campaigns hold more than one message. Requires the Campaigns feature and access to the campaign; a campaign that is past the point where its messages may change is a 409.