The way I see it, if you got the NTT stable from the LFS or somewhere, there's a good chance it will be stable the rest of its life. That's how it has been for my 3 NTTs. The same applies to unstable ones. I got a 8-9" NTT and its been unstable with washed out bars since I first got it 2 years ago. Its not aggressive and a voracious eater but still unstable. It's the only unstable dat I got in my 100 gallon.
With IT, I think they are more susceptible to being unstable. I got an 8" IT in the same sank and its stable most of the time. During water change, it becomes a little bit unstable and it darkens up just a tiny bit. After the change, it goes back to is stable self. It's also partially blind. I think it has blurred vision because its eyes have had a haze on it the past year. Its as fat as a pig though