Thanks for good question. You’re spot on. This is actually a core pattern in Trunk-Based Development often called Branch by Abstraction.

Instead of keeping the new architecture in a separate Git branch for months (and facing a merge nightmare later), we keep both versions in the main branch. It might feel ‘messy’ to have two versions of a service at once, but it’s much safer.
Using a feature flag allows you to:
Merge small, incremental pieces of the new architecture daily.
Run ‘dark launches’ where the new code is executed but its results are ignored/compared with the old one.
Flip the switch instantly when ready.
The key is the ‘cleanup phase’ — once the rollout is 100% successful, you must prioritize deleting the old code and the flag to keep the codebase clean.”

Based on your comment, I added a new paragraph to the article directly below the heading “What About Large Architectural Changes?”