It aims to provide a definitive and less buggy experience, including backports of some modern conveniences (such as item smearing and ctrl sprint) and retromods that didn't exist back in the day.
Rewind packs are compatible with the major FTB pack from their same version. You can load an FTB Ultimate world in Rewind Upsilon and it will upgrade seamlessly. You just get a less buggy and more featureful experience.
So, why? It's pretty simple: these old versions of modded Minecraft are quite
unique on their own, especially compared to modern modded, and the experiences
offered by them still hold up today. Rewind packs are for anyone that wishes to
revisit older versions of modded Minecraft, nostalgia or no. Playing a Rewind
pack shields you from many known bugs, ranging from mild inconveniences, to game
crashes, to dangerous remote code execution vulnerabilities. Additionally, all
the mods are fixed to work on modern Java 8 and disable all reliance on
Upsilon is Rewind's flagship pack (and first pack), built for Minecraft 1.4.7.
It is also the first ever modpack to use NilLoader.
Upsilon is not quite ready for release; watch this space.