"It can be phased in, like:
if (blocknumber > 115000)
maxblocksize = largerlimit
It can start being in versions way ahead, so by the time it reaches that block number and goes into effect, the older versions that don't have it are already obsolete.
When we're near the cutoff block number, I can put an alert to old versions to make sure they know they have to upgrade. "
This is a hard fork. I don't see anything anywhere about a SegWit hack, or a soft fork as they call it. The nodes who wouldn't upgrade would split themselves from the chain.
Bitcoin was always supposed to hard fork into a version with a larger block.