If you have a codebase that big, can you even fit enough of it into a context window for the LLM to make correct and meaningful changes across all of it? Admittedly I've only used LLM-based coding for smaller projects.

Such repetitions can regularly be deterministically automated, like find -exec sed and similar medium level tools.

If you spend a lot of time performing monotonic tasks, then your organisation needs to delete and refactor for a while until change in 'hot' areas of the code base are easy to make. Reaching for some code synthesis SaaS to paper it over will worsen the problem and should result in excommunication from the guild.