The world starts again. Everything built, owned, earned or carried on the old one is gone - it is kept on the server, but nobody plays on it any more.
Everything the harsh mods add now exists past the line and nowhere else: their keeps, their ore and their weapons in chests. The safe half is Minecraft with machinery in it.
Your game needs nothing from you. The pack updates itself when you connect.
The full list of changes
01Why the world had to start again
The rule that keeps the harsh mods past the line is applied to ground at the moment that ground is first made, and a chunk keeps whatever was in it the first time. Applied to a world people had already walked across, it would have held for everywhere nobody had been yet and changed nothing about the rest.
So the half people build in would have gone on holding boss keeps, seams of the far side's metal and rifles in its dungeons, for as long as that world lasted - while every new place was correct. One world with two sets of rules in it, and no way to tell by looking which ground you were standing on.
The old world is moved aside rather than deleted, and so is the old map. Neither is coming back as a place to play; they are kept because a morning after a wipe is exactly when somebody asks what the old place looked like.
02The far side is the only source
The keeps and arenas of the mods that exist to be frightening no longer generate on the safe half. That rule was written some time ago and had never actually been switched on here: they had been generating on both sides all along.
Their ore goes with them. Anthralite, sulfur and phosphorite are past the line now, which is what the respirator ladder always assumed - the near band is breathable in a shirt, so the metal for the first mask is a walk rather than a wall.
And their weapons come out of their chests. A dungeon, a mineshaft, a stronghold, an ancient city, a bastion or a buried treasure on the safe half is vanilla loot; the same chest past the line is not.
A village on the safe half keeps its gunsmith, and his chest is empty. The house is put there by the mod itself in a way nothing can take back out - what can be answered is what is inside it.
03Two things that used to arrive on their own
The gun mod's own raids are off. Eleven of them, every attacker carrying a firearm and a set of armour, scheduled at night at whoever happened to live nearby. A gun in this world is meant to be something somebody built or carried home, and the same rule already forbade ordinary monsters from spawning with one.
The heavy armour sets no longer appear in ordinary chests. They were never craftable here on purpose - they are what somebody comes back from the far side wearing - and they had been turning up in dungeons and igloos and village smithies anyway. That door is shut; the cache past the line is the one that is open.