Hello, I think I can answer that since my work and Rosedale coincides, including working with people with similar strategies.
The answer is a bit tricky. If you have been on a high protein regimen and use high-intensity training, going on medium to low protein regimen will result in a temporary muscle loss. It is due to a healthy reduction in IGF-1, insulin (assuming you are not insulin resistant), and probably testosterone too. Also, if you exercise, you will need to avoid reaching into failure and avoid muscle damage.
(Continue) Most people with average diets, won't experience a difference in muscle mass but they may lose a bit just in the beginning until the body becomes accustomed to adequate protein. Then, it will stabilize. In fact, the body will become very good at maintaining the exact lean mass at all times regardless of the environment versus other people who are more likely to fluctuate.
Having said that, the older you are, and the bigger the metabolic problems that you experience (high blood sugar, etc.), there is a good chance that you will GAIN lean mass – sans exercise. I have seen this several times. This is due to improved insulin sensitivity and the halt of burning muscles and bones as a fuel. I guess that everyone over age 60 will experience that.
Thank you for this update. Nice insight and explanation. My concern was more on the diet side, and how to keep the protein under control, since the foods I eat for high fat also have high protein. I am reaching 100g almost daily, and my target should be around 70g. So still a lot above the target...