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, items and entities are presented as missing textures or as Source Engine ERROR models

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