A couple of things:
The way you describe it (reformat), my guess is that you didn't do and "update" but rather a "clean Install".
This is the procedure where you install the new OSX over an existing OSX. It usually leaves the previous data untouched and only replaces the data that is necessary for the upgrade.
This is where you format a drive and install a new OSX version on an empty drive.
My guess is that you had data from an earlier GarageBand installation on your drive (legacy). When you reformat the drive, you would erase that. Unless you have a backup from where you can copy data back to your new system. A good practice in general is to clone the current system to an external drive before doing a system update, just in case something goes wrong or you remember a few weeks later that there was an important file that you forgot.
The only way to get your legacy instruments back is if you have an installer disk of the old GarageBand with those dinstruments or the Jam Packs in case you installed them separately.
Hope that helps
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'
So for some reason it just completely got rid of the old Garageband this time around.
If GarageBand '11 came preinstalled on your Mac with Mt. Lion, it might still be showing at the App Store in your "Purchased" tab. If yes, try to reinstall GarageBand '11 from there to get the "Legacy" instruments back.
And have you tried the in-app purchase? Since you previously owned GarageBand '11, you should be able to get it for free, using the "Restore" option from the "GarageBand > Application" menu, see: Expanding GarageBand for Mac (v10.0) content with an in-app purchase