If this is true, it leaves only one question: why would anyone buy such a high-profile account? Maybe that's the reason he stopped posting in English too, but if the account changed owner, that can't go unnoticed.
Imagine someone else using my account: how long would it last before it's exposed? I hope it doesn't even take a day!
I don't know if you know this, but high profile accounts with positive feedback are very valuable and can be sold/purchased for a lot of money.
Maybe that is the reason why he changed his username and sold it with new name instead of old Ratimov.
This transactions all be done outside of bitcointalk forum, so nobody would have any proof that he did anything.