It's interesting that the author went with this route, which is plausible as it can affect Ryouma is his new body and new environment. I personally don't think it is a form a regression but an adaptation to the new variables. Neurologically, if the brain structure stays the same after migration, it won't affect Ryouma as much in the beginning (I can't put up good solution for fitting bigger brain to a smaller body, so there might be loss in neurons/memory). His new body's sensory input/output, body control, hormone levels etc could affect him in long term, and new environment that can affect him such as circle of community he interacts, food he eats, etc.