Access list entries (using true IP vs. NATted address) and NAT (significant changes) syntax changed after 8.2.
The command parser will convert the sysntax automatically but it sometimes fails to do it correctly. It does generate an exception log that you can examine and address the issues.
Most people are better off doing a manual method where they both convert the entries as well as clean up any unused and invalid bits.
All of that aside, why bother on an old 5520? Those devices are well past end of sales and you cannot upgrade them past 9.1. the added features from 8.2 to 8.4 or even 9.1 are probably not anything you will be needing if you gone this long without them. You would be better off with a current generation 5516-X that can run the latest and most secure software versions and have Cisco support for years going forward.
