I filled my TC with wood and stone and hit auto-uprade. Occasionally, one material would run out, and I'd add more. Very large base started as 95% thatch (I upgrade some parts to wood), and I allowed auto-upgrade to do most of the work. I supposed you'd have to build a massive base from thatch and occasionally allow one material to run dry, occasionally refilling. If you'd like, you have my permission to test anything you want in my base. It's possible this downgrading would happen again if I put wood in my TC. Feel free to mess around.