Interesting - made me go look! So I am now on 3.1. I have simple weighted disabled. On my old courses, the gradebook is still set to simple weighted and simple weighted even shows in the aggregation dropdown. On the new courses, only Natural is listed. My old courses are hidden but that does not "lock" a course, just hides it from students...
I believe though if I switched an old course to Natural, I would not be able to switch it back but I am not about to try that and mess up an old gradebook!!