Well, changing your domainname isn’t that bad if it is an improvement above the old name. I understand why he did it: he will always be blamed his site’s success is drifting along the success of the bigger brother Club4AG and it is limiting to the cars with a 4AC engine only, just like AEU86 is limiting to the AE86 only.
Currently he actually implemented a redirection of the old content to the new domain with a 301 HTTP header (Moved Permanently). This is indeed the correct way to do, however on the new domain we get redirected to IP.board instead of the good old phpbb (behind /old/ ). Of course we get an IP.board error: “Sorry, the link that brought you to this page seems to be out of date or broken.” which will cause Google (and other search engines) to end up on the same page for every already indexed page. They will mark the content duplicate and remove the old pages from the index.
Also the second page I requested ended up in a site overlay which caused me to click away from the site: IMO people doing site overlays should not deserve traffic!