I don't think Safe means "can't decrease level", as I had Units decrease levels even at Safe. Decreasing level is related to Risk afaik, and it seems Rings don't have any Risk listed when you go upgrade them.
When upgrading Units, I had quite an easy time going through Safe, but for the Ring, I failed 6 times out of 11 attempts. That's either too unlucky or wrong. If you're gonna throw a low % rate of upgrade, at least change the Safe wording to something that reflects the actual low chances.