The changes aren't as drastic as you think, for the most part. There is some material up on the official USMLE website. But you do realise that all preparatory material is based off what's on the actual exam, so it's only after they start administering the new format that UWorld will be able to provide material that's "very close to the real thing". Until then, they'll probably base their material on the description of changes. When they'll do that though, one can only speculate. Could be right after September. Maybe you could write to UWorld and ask them?