Related to this post, we're going to take a closer look at classes ( and jobs ) and go over some of the common scenarios for code upgrade. The LCS Code upgrade tool will do its best to intelligently upgrade and convert your code but it will need your help a lot of the time. First, the tool is saving you tons of re-work. While it may get it wrong some of the time, its still doing lots of work. You will still need a 2012 instance with the code base available for reference.
Upgrading code from AX 2012 to D365 can be a daunting task. MS has given us some tools to help but we'll go through a high level overview of the available tools and methodologies to get the process started.