To be honest, the case is still the original one, but almost every other part has since been replaced. Now, I’ve taken it back to the shop where I bought it 20 years ago and asked them to upgrade the motherboard, CPU, and memory - the last of the original parts.
So, is it still the same computer?
I also like that I can just keep replacing parts on an existing product rather than buying an entirely new device each time. That’s exceedingly rare feature these days.
Id consider mobo+cpu swap to be a new computer, and rarely if ever do just a cpu or just a mobo swap. Most other things id consider to just be moving the same build to a new case or an upgrade.