That's why change orders should happen for anything that isn't specified at the time of signing the contract. They can be zero cost, they can be clarification

That's why change orders should happen for anything that isn't specified at the time of signing the contract. They can be zero cost, they can be clarification change orders, they can swap scope, or they can involve additional cost. I try to get to a cadence of about one change order a week on projects lasting longer than ten weeks. Sometimes, depending on how onerous getting an SOW signed can be, we'll even queue up change orders (with the client's willing participation) to execute right after the SOW is approved by legal. There are plenty of apps that version mockups, but it's also possible to add layers to existing documents as well, whether it's photoshop, visio, etc.