The first thing i would do is to call into support to confirm that the hardware will not support the latest version. A very select few of the E2000's were listed as unsupported even though they actually were.
Second, most warranties are nearing the end for the older E2000 appliances, so it may just as well be worth it to uprade the appliance.
As far as restoring the config, v6.7.1 can import a 6.5.4 backup, then from there you can upgrade to 6.7.2. If you do decide to go with new hardware, then be sure to request 6.7.1 otherwise you will need to re-image the appliance.
If you need to re-image the appliance then you can go to: