you should not load the config file of an older fw to a newer one
I think that's correct for the DNS-323. Since it's a binary file, I have a bad feeling that it's just a dump of the config area, and any changes in format would screw things up.
Many devices dump an ASCII file, which you can normally restore to a different firmware level. My Actiontec MI424WR router and the D-Link DNS-615 router both output ASCII files that can be used to reconfigure a different release.