I’m working on having a large scale open source project support a new SQL database (Postgres) and the migration work is about 2500 lines currently since I have to touch basically every part of the project. I don’t want to do the migration piecemeal since turning on the Postgres unit tests for only sections of the code base and keeping track of that and worrying about all the dependencies of code changing as other people touch the project is a lot more overhead than doing it all waterfall style.

Now, I will say that these type of things are not everyday kind of PR’s, but they have certainly happened several times over the course of my career. Probably once or twice a year if I had to take a rough guess.