All true. Add new bolt face (whole bolt) for different size cartridges.
To go 243 to 284 you would possibly need a different magazine due to the larger case. But that's IF you want to run it as a SA.
To run as a LA (most would say do this I believe) you'd just need a new bolt stop. $50 and you can be all setup. No smith needed.
Can use same bolt in that case.
I'm building a 7wsm right now and if I didn't have brass, dies, etc for that I'd go the 284 route for sure. It's appealing loaded long in a tikka.