Are you saying that ALL books on Amazon JP are still in the old format? Of just the couple that you tested?
You need an address to register, but it is not checked if this is your address is as long as it exists somewhere in Japan. It could be a school or public buildings address it seams, at least this was the case when I researched this issue a few years ago.
I could make an account back then with a family members address in Japan but really any address would have worked as well.
The only inconvenience is, that you have to disconnect your kindle when changing accounts and that means you also have to load the dictionaries again when doing so.
One interesting thing I found out is, that accidentally I changed my German account to the same email address as the Japanese recently and thought that might have messed up everything, but if I log in on the Kindle the account is chosen according to the password I type in.
1 Like
Maybe I can use the imperial palace as my address and then troll the 天皇さま by ordering some eroge his way? 
2 Likes
Makes me wonder what address it has 
1 Like
〒100-8111 東京都千代田区千代田１−１
3 Likes
Of three hundred fifty I have tested over a five year period, 20 of which were over the past month, amazon jp has used the old, crackable format 
7 Likes
へへへへへへ！！！
うらやましい
Use a different Windows profile for your Japanese Kindle account if you just want to DeDRM them, and then move the output files to a central location? No fuss, no muss.
1 Like
I don’t have quite the extensive testing that you have, but I am using Kindle version 1.16 and the latest DeDRM and I haven’t encountered any US books that I couldn’t subsequently open in a different reader.
1 Like
Good to know. It’s extraordinarily annoying that amazon US doesn’t let you swap formats because I know it’s malicious and I know it’s going to screw me over at some point.