Shane,
Appliances will refuse to be added to a cluster if they are not all on the same version. Configs have different options and it is beyond complicated to even try to account for differences so all the same version is a hard requirement.
In order to be able to backup a config using saveconfig, you have to completely remove a given appliance from a cluster first. The centralized management feature adds so many different things to a config that you will not get a coherent config otherwise.
Cisco can send you a box with an older build upon request but too many versions can delay your RMA since they can end up having to pull a specific image out of the archives.
I would be very careful about trying to load a config from an older version. It may or may not load even if you edit the version information in the file. The problem is that during the upgrade process things are done to the config file (adding new options/removing old) that don't necessarily get re-done later on. I've done it in the past but not without a stern warning from support that a resetconfig may be the only solution if the box has problems later.
My sincere recommendation is to take the new box, add a minimal config to get it upgraded to the latest & greatest stable version of AsyncOS and manually add your setup. You will be much better off in the long run with an appliance that started from a known good place.
For the future, when you are upgrading from a really old version (something I have done many times) check the upgrade path to make sure that you are dealing with a supported upgrade. Even then, you are going to be better off doing an upgrade in multiple steps rather than jumping really far in one big upgrade. Do a saveconfig at each interim version so you can revert back if needed.
Whenever you upgrade a box in a cluster it is always safer if you pull it out of the cluster completely and do a saveconfig first. Then once all of your boxes are upgraded you can rebuild the cluster from scratch. I realize that is the 'hard' way and usually not necessary but it all depends on how well you can tolerate a down box if something goes awry.
I will be the first to agree that manually redoing a config is really a pain but if nothing else it will refresh your memory of all the more subtle configurations that you have set up. I always looked at an RMA box as a "fresh start" and did a manual reload unless I had a known good config.
