I’m starting a new topic since this was closed by @sybren :
Introducing a switch between Z=up and Y=up would require a massive development effort. […] All in all this would be a huge time sink for no particularly good reason (exporters can already change coordinate systems; for example the Alembic exporter already exports to Y=up because that’s what Alembic uses).
Since changing the x/y/z coordinates system would be too much effort for too little benefit, I’d like to suggest a much simpler alternative. Can you make a setting for changing the colors of x/y/z in the editor? This would be useful when working with models that are meant to be exported to tools that use a different coordinate system.
For example, if the model will be exported to Y-up coordinate system, I could set it so Z (up) is green (so that I know it will become Y) and Y (forward) is blue (so that I know it will become Z). The editor will still treat x/y/z no differently than before, but the user will know to mentally associate the colors with directions (instead of letters), so the colors match the ones in the target program.
This is a good compromise that is still enough to help users working with external programs, while being much simpler to implement, since it’s just an editor “cosmetic” change.