Upgrade from AX2009 to Dynamics 365 for Finance and Operations

Hi. I'm looking for the way to upgrade from AX2009 to Dynamics365 for finance and operations.

Actually I cannot find materials about this stuff. Also, the customer has lots of customization codes in his site. Therefore, I want to know the method to upgrade their source. Can it be done directly from AX2009 to Dynamics 365? Or should it be transferred through AX 2012? For example, AX2009 source can be migrated into the AX2012R3. Then, it can be migrated to the Dynamics365. Is it essential or it can be omitted?