Can grey knight land raiders be upgraded with the power of the machine spirit? I was thinking this because the deamon hunter codex came out before the spacemarine codex.

Well according to GW's Armies of the Imperium (which includes V4.0 Errata), the answer to that would be no.
I would assume that is because IF it did have one, it might run the risk of being possessed?
I'm sure I'll get put in my place with the right answer soon enough

According to Pete Haines, GK Land Raider Crusaders use the same profile as SM Land Raiders Crusaders, and all other units/wargear that had the same profiles before use the updated profile now.

> #1 - Should the rules for the Land Raider Crusader from the Space Marine codex replace the entry for the Grey Knight Land Raider Crusader from the Daemonhunter codex? Infernal Device on Chaos Land Raider - work the same?
> Also for other codexes where landraiders are taken as transports (DH, WH and Chaos), do these idential vehicles count as scoring units as well?

Yes

and

9. Do all "previously-identical" items used updated stats? This is particularly important for Daemonhunters w/ Assault Cannons. You specifically addressed 2 examples (Storm shields and teleport homers) so I believe logically we can conclude that the same rule is applicable across the board.

A: That’s true. A Grey Knight Dreadnought or Land Raider Crusader uses the new assault cannon stats. Don’t make any assumptions about psycannons though!

It's the second quote that is more useful (as the first references the LRC), as it states that all items that were the same continue to be now the new codexes are out.

However, this was an unofficial FAQ.

I think the Wargear book does state this though - but I don't have it myself to check.

Spack wrote:I think the Wargear book does state this though - but I don't have it myself to check.

According to the icon guide "each vehicle upgrade and wargear item is accompanied by an icon showing which codex it originated from". The icons next to "Power of the Machine Spirit" are Space Marine and Witch Hunters/Demonhunters. So I would concur with the GK LRC being equivalent in all ways as the 4E SM LRC profile, with the same options being available excepting any GK specific modifications.