FAA Letter of Authenticity which expired at the end of August.My plan now is come over to the US in January'15
Read it again and I think you can answer your own question......
The purpose of the Letter is to verify you hold a certificate and a medical which are not expired, surrendered, suspended, revoked or otherwise invalid.
The letter itself is valid for 6 months and anything you wish to accomplish needs to be done within those six months.
You need to apply for a new one with the FAA as the CAA has bugger all to do with it but I do believe you need to pay them.