Hopefully it's common knowledge that Revit is not backwards compatible. With The 2014 version now out and available people will want to upgrade any custom families they have to avoid the dreaded "This is a one time only upgrade process" dialog.
Your friends at Autodesk have thought about you and hid a gem of a BAT file in the install folder to help you out. Look for the Utilities folder then open the ContentBatchUtility folder.
In there, you will find a readme with these instructions.
To Use the Content Batch Upgrade Utility:
1. Copy Upgrade_RFA.txt and Upgrade_RFA.bat into the root directory of the library that you want to upgrade.
2. Run the Upgrade_RFA.bat to create the file list to upgrade, famlist_rfa.txt.
3. To launch the utility, from the library directory, drag Upgrade_RFA.txt onto the Revit icon on your desktop.
4. After upgrading, delete all backup files from the library.
Make sure all of your .rfa files are not read-only and delete all backup files from the library.
If a particular family fails to upgrade properly, the utility will stop.
If this occurs, open famlist_rfa.txt in Microsoft Notepad, and remove all previously upgraded families and the failed family from the list.
Save famlist_rfa.txt, and re-run the utility.
Once you drag Upgrade_RFA.txt onto your Revit 2014 icon, it goes into action meticulously opening, upgrading, and closing each file in the folder structure. Believe me, faster than a person can, and a lot more fun to watch than do.