The requirement for renovations will vary based on locally adopted codes and their interpretation by building officials. In more affluent areas, the tendency will be to require upgrades along with relatively minor work. In less affluent or rural areas, the tendency will be more libertarian - i.e. building codes and their enforcement are subject to the political process.
Hardwired alarms are also required for major renovations. However, you basically have to take it down to the studs before this requirement is triggered.