many people have been complaing about not being able to use mods after updating, due to losing there old .jar in the update. well, if you own minecraft, theres a way to get AT LEAST the previous version, possibly ones from sooner periods. its called the tekkit launcher.
tekkit itself is a modded version of minecraft, with its own launcher that allows you to choose diffrent versions of there modded monstrosity. however with the way they have set it up, the .jar in the tekkit folder is completely vannila. the way this is all legal is that tekkit launcher requires you to log into minecraft.net, and if you haven't bought the game, you cant download the tekkit. much like standard minecraft

after downloading tekkit, go into your usual minecraft directory (assuming you dont use the profile manager mod) wich is typicly in appdata/roaming. before going to .minecraft, you'll notice a .techniclauncher folder. click that instead. go to tekkit (or technic, depending on wich pack you chose) then bin, then copy the minecraft.jar into your.minecraft folder (preferably one used by a profile manager, so you can have multiple versions at once. the magic launcher is another good choice)

there you have it. if you get confused by the tutorial, drop a comment and i will try to help