OS Sierra, a huge format change? speed?

[rich in distress]

Of course they are pushing. It's not only the best OS in the world so far
It is also a transitional OS for what will actually be a a huge "disruption in the force": the new file system that will be introduced shortly.
I tell ya, if you don't care much about the OS, you would be better equipped with Sierra than with any other.
Fear not the swaps, fear not the bandwidth. Just do it while you have a nice backup.