I have been using version 1.8.23.1 as ALL the later version crash on startup.

Here is a copy of the crash-report if it helps.
crash-2023-07-11_19.43.38-client.txt

The modpack I am updating is FactoriaCraft-ServerEdition.

Try using this alternative version of SecurityCraft. If it doesn't work initially, make sure you have a mod such as MixinBootstrap, Phosphor, or another mod that loads mixins in 1.12.2. Let us know if it works or not. 🙂

EDIT: that ninja. See the links above as well.

Might be a duplicate of #300. If the solution there does not work, try also installing https://www.curseforge.com/minecraft/mc-mods/mixin-0-7-0-8-compatibility.

All works. Thank You