Sharing blog post which I wrote on company blog site about

A. How to plan upgrade from AX 2012 to Dynamics 365 for finance and operations

B. Upgrade Code from AX 2012 to Dynamics 365 for finance and operations 

https://global.hitachi-solutions.com/blog/how-to-plan-for-an-upgrade-from-dynamics-ax-code-to-dynamics-365-for-operations
https://global.hitachi-solutions.com/blog/upgrade-dynamics-ax-code-dynamics-365-operations