When the regulation requires that modifications bring things up to the new code and you've got decades of regulations like that piled up the most reasonable options are touch nothing or rebuild the entire house
In Indiana, for example, you don't have to upgrade the electrical system if you are adding insulation, replacing windows, or replacing the roof.
You may or may not need to upgrade your electric to replace a plug, mostly because some older systems aren't grounded. It is possible to rewire a single room without upgrading the entire house.
You don't need to upgrade the plumbing while you upgrade the electrical, just like you don't need to upgrade the electrical if you are treating the asbestos (a problem my parents had with an old house built in 1918). Most times, whatever you are fixing needs to be up to code but not everything. If it weren't reasonable, it wouldn't be prudent to buy an old house without detailed upkeep and upgrade records.
