Best AI Prototyping Tool for Iterating on an Existing Product?

Most conversations about the best AI prototyping tool seem to focus on launching brand-new products from scratch. What I'm more interested in is how these tools fit into an existing product that already has users, established workflows, and years of design decisions behind it. In that situation, the challenge isn't creating something new—it's testing improvements, exploring interface changes, validating ideas, and communicating updates without spending weeks rebuilding screens manually. For teams using AI p