If this is a breaking change to a major upgrade path, like a major base UI lib change, then it might not be possible to be broken down into pieces without tripping or quadrupling the work (which likely took a few folks all month to achieve already).
I remember in a previous job migrating from Vue 1 to Vue 2. And upgrading to an entirely new UI library. It required partial code freezes, and we figured it had to be done in 1 big push. It was only 3 of us doing it while the rest of the team kept up on maintenance & feature work.
The PR was something like 38k loc, of actual UI code, excluding package/lock files. It took the team an entire dedicated week and a half to review, piece by piece. We chewet through hundreds of comments during that time. It worked out really well, everyone was happy, the timelines where even met early.
The same thing happened when migrating an asp.net .Net Framework 4.x codebase to .Net Core 3.1. we figured that bundling in major refactors during the process to get the biggest bang for our buck was the best move. It was some light like 18k loc. Which also worked out similarly well in the end .
Things like this happen, not that infrequently depending on the org, and they work out just fine as long as you have a competent and well organized team who can maintain a course for more than a few weeks.