Helps to have some idea of what you might or might not have assumed. As you say "bringing parts of the system down, or living with multiple versions of code in

Helps to have some idea of what you might or might not have assumed. As you say "bringing parts of the system down, or living with multiple versions of code inside your system" — at least momentarily some other modifications to data will be paused. In practice, the data is probably in a relational database and that will determine how the transition is done. Here's an experience paper from long ago: https://news.ycombinator.com/item?id=31162816 https://news.ycombinator.com/item?id=31162816 ---- When everything is within Smalltalk — create-update-delete methods to change behavior; to change state, find all instances, copy data to new objects, magically become: all references to reference the new objects instead of the old objects, check that GC removed the old objects, and save. These Computer History Museum videos might show something like that: https://computerhistory.org/blog/introducing-the-smalltalk-zoo-48-years-of-smalltalk-history-at-chm/ https://computerhistory.org/blog/introducing-the-smalltalk-z...