Honestly, what I reckon they'll do when B/W comes out, is they won't change the rules OR have a mid-season rotation. Unless they make a huge mistake, they have two options:
1) Release B/W but not to bother with changing the rules or the format.
2) Release B/W and change the rules and the format to HG:SS-Onwards.
Changing the rules to the new B/W rules in Japan without rotating out some earlier sets to make it HG:SS-On would be a disaster, although changing it to HG:SS-On and keeping our current rules wouldn't be so bad, but doesn't really fit in with the "theme". So I reckon they'll do one of the two things I just mentioned.
1) Release B/W but not to bother with changing the rules or the format.
2) Release B/W and change the rules and the format to HG:SS-Onwards.
Changing the rules to the new B/W rules in Japan without rotating out some earlier sets to make it HG:SS-On would be a disaster, although changing it to HG:SS-On and keeping our current rules wouldn't be so bad, but doesn't really fit in with the "theme". So I reckon they'll do one of the two things I just mentioned.