you are making things very complicated here.
you can only call a "hard fork" an "upgrade" to explain what it does. but to not call it a "hard fork" at all means you are removing the other characteristics that it has.
the fact is, there isn't any clear definition of a "hard fork" and "soft fork" when it comes to blockchain based projects but generally speaking a simple definition is this:
whenever you add/remove/change a new consensus rule you are creating a fork of the project. it is a hard fork of the changes are not backward compatible and they are considered soft fork if they are backward compatible.now regarding chain split since it was mentioned,
both hard fork and soft forks can lead to a chain split because you are changing consensus rules and people can decide not to follow them and remain on the previous chain and reject the new blocks.
If upgrade comes from original core team with totally consensus, it should be called as upgrade.
Instead, if upgrade results in un-consensus, and the original core team or community divide into several parts with different visions and plans for developments after the update, it should be called fork.
wrong. it doesn't matter where the "changes" come from and more importantly "consensus" means the whole network agree on the change not the (original/core/ether foundation) developer team accept it!