Why did Sirrus and Achenar abandon the plan to extract memories from Atrus' head to learn The Art to form the new plan which was to go to Haven and Spire?

Anyone?

I think they were planning on extracting his memories as a sub-plan. They were figuring out parts of it as they were looting other Ages. So they didn't abandon the plan, just ended up getting stuck in their "sidequest" looting.