So about nine days ago...looks like the diet change is sitting well with him!
Here's what I'm thinking: Sometimes a long cycle (lasting more than 12 hours, or a very late nadir) is just one of those things and doesn't mean anything. The next cycles will all be completely normal. Other times a long cycle is an indication that the dose is too high.
The numbers you've been getting (reds and blacks with little change at nadir) would indicate that today's cycle is just random. However, the recent change from high carb to low carb food is a significant factor and can, in some cats, lead to rapid drops in BG numbers, which might indicate that today's cycle is not random and does mean that the dose is too high.
Lacking a crystal ball, I don't know which one it is. Only time will tell. You're still in higher numbers, so assuming a regular AMPS, if you decide to reduce in the morning, I wouldn't drop too far. The more time he spends below 200, the more chance his pancreas has to heal.