yes in the first episode the defiant appears sisko explains that the defiant prototype was 'mothballed' when the borg threat waxed then waned perhaps they kept the registry as is as a temp measure or to obfuscate viewers as well supposedly while on training exercises the valiant with crew and cadets was somehow disabled attacked i think command and other officers crew were killed and cadets were then in charge but didn't know how to repair the warp engines sufficiently big mistake