As the t9 ramps down, the s9 ramps up, you're essentially losing nothing (and gaining a smidge as the s9 is "better").
What happens is that your change in hash rate takes 5Nd.
So if your rate goes down to 0, it takes 5Nd to get to 0 reward from your current, but if your rate goes up, it also takes 5Nd to get to the new rate from your current, which at the moment is just under 7 days.
Gotcha...so I power down a 10.5 th/s machine and power up a 13.5 th/s machine, 3th/s has to start at the begining of the ramp, but 10.5 maintains it's current status. Diff adjustment not a factor, but only a means for the pool to convert hashrate to share rate?
You got it...the pool will see the worker as the same machine that suddenly got stronger and auto-adjust the diff.