Depending on the nature of your product and your rewrites in my experience a customer facing system that has parts improved in parts can be interpreted as not solving the right problems (if something UI related is being updated).

The best thing I could say about it is that when approaching behind-the-scenes pieces this is my preferred approach, but for customer-facing pieces I prefer (and have had better reception on updates) to update those pieces as a whole. But again, this depends on the complexity, updating a small piece such as a button should not require a large change but updating the layout or styling would require more significant updates.