1- Just make an us save.
2- Using Multiman or anything similar go to the saves folder and copy the contents of the jap save (BLJS10093) to the new us save you made (BLUS30903).
3- Load the save (It will load then became corrupted)
4- Save again and delete the corrupted save if you want.
Make sure to make a backup first. Also note that Evil + Chaos will possibly disappear if you unlocked them (So make sure the Jap save is not saved in one of these difficulties or you will face some problems). But you can unlock them again in Newgame+ so don't worry. Also, the attachments (melon, goggle, etc) will disappear also but these are not important.