Without chronographing, it is speculation. In theory, a lighter case has more volume, so less velocity. But in reality, you would have to check the relative volume between the 2 cases to be sure. And there may be a few other factors that affect the velocity. It is likely that the velocity would not be much different, but it's impossible to say "how much." Naturally, one should drop down a bit on the powder charge when changing components like the case and work back up.
