Leo was making the point that your post was duplicated (this one = #12144276 and the other is #12144271). It seems to be a common issue with the new platform they migrated CSC to a couple of weeks back.
Anyhow... to your question:
Assuming you have the necessary memory in your 5520, it is recommended (required actually) your do an intermediate upgrade. Please refer to the Release Notes which recommend you first migrate to 8.4(6) and then to 9.1(x).
Part of the process in that first step is to parse the old configuration file and translate the NAT syntax. That's done automatically by the ASA when reading in a pre-8.3 configuration that uses the old syntax for NAT (and a few other things). Some people prefer to re-write the NAT section themselves, using the opportunity to revalidate and possibly clean up their current configuration. That first upgrade will generate a text file on the ASA with any errors in the parsing process indicated. You should examine and, if necessary, resolve those before taking the step up to your target 9.1(x).
If you're working with a partner, there is a tool they can use to do that translation offline for analysis outside your porduiction environment.
Be sure to have a currrent backup of your old configuration (including any pre-shared keys, certificates etc.) before beginning.
