FlashFreeze

FlashFreeze lets you remove and add content- adding mods without dropping any blocks, entities or items.
 

You can use it to migrate to a newer version of Minecraft without waiting for That One Last Mod, or turn content mods on and off one by one to nail down that pesky bug.

Usage

All unknown blocks are presented to the client as bedrock, unknown items - as unusable nether stars with special NBTs, entities - with noGravity armorstands.

Once you turn the content mods back on, all the FlashFrozen™ content is defrosted 100% fresh!

FlashFreeze works purely on the server side, so you don't need it to join a FlashFrozen™ server.
You can install it on the client if you want to FlashFreeze™ a single player world.