Here’s the reply I got from support: "The 30 day money back guarantee is only valid once. Whether that's Pro or Pro Unlimited, they share the same refund policy. So if you have previously purchased Pro, you will no longer be entitled to the guarantee if you purchase Unlimited."
Surprised this idea isn’t more popular... This would generate a lot of extra revenue for SoundCloud while helping to directly promote individual artists so it seems like a win-win to me. Promotion could also or alternatively be on a per album or artist basis.