Here's my opinion of how it went / will go down.
Grant and Koston are friends and work together on a lot of projects, Grant also films for Nike.
Koston and Guy are starting a brand, but it takes time to start a brand.
Grant came up with the idea of Koston riding for WKND in the interim, offering him a pro board which would sell and he would be able to collect royalties. Also this will increase demand for WKND and allow them to start ordering more boards. Basically a jumpstart to their already strong sales. All in all it's a favor from Koston because Grant benefits way more than Koston does.
I bet Koston will have several WKND boards.
Once Koston and Guy's company is fully running, Koston will leave WKND and Grant will make another funny video about that.
WKND will have a little increase in brand loyalty so their sales levels will remain higher after Koston leaves.
Also once Austyn is fully on there will be a video about that. I hope they tie in the Koston / Austyn thing, could be a funny video.
Austyn Prediction: Rest of the year to film a full part. Koston leaves. Austyn on + part drop.