Old iCloud Apple ID on device but new Apple ID on iCloud.com

Old device iphone4 version 6 ,new device iPhone 6 plus version 8.4.

BEfore I restore backup of my iPhone4 from Itunes, iCloud of device and PC are match. Same apple ID. After restore backup from I tunes, the iCloud of device change to old Apple ID which is no longer use and password is forgot, but the iCloud of PC still remain with my new Apple ID. From the icloud storage of device, is showing both iphone 4 and iPhone 6. But when I backup my iphone6 to iCloud using old ID, the photos of the iPhone which I restore from iTunes didn't backup to iCloud PC. I try to log out iCloud from my device, but fail because without password, Reset new password but only sucess to change new Apple ID password.

HOw change I sign out my iCloud (old ID , forgot password, fail to change password) from device and log in iCloud of device using new ID. Thank you very much.