We just recently went through this and I rebuilt the config from scratch. My config was only about 6 pages so it wasn't terrible. This also allowed for some consolidation into object groups and auditing of rules that existed on the ASA.
If your config is too large you could try setting up 8.2 in GNS3, applying your configuration then updating. Never tried an ASA upgrade in GNS3, but have had 8.2, 8.3, and 8.4 working properly in GNS3.
Another option, if you have an Active/Standby pair, would be to break the pair during maintenance and upgrade the Standby. Once everything validates, then make it the primary and bring the other unit back into the pair as standby after upgrading it. If testing fails, downgrade the standby unit and bring it back into the old pair.