AX 2012 R2 CU9 (Originally CU6) to D365 F&O Code & Data Upgrade

 Over the past several weeks I have been attempting to upgrade our AX DB into D365 mainly using instructions from here.  

Before doing the steps there, you will need to see if you  have a 'Display' menu item named 'SysCheckList_UpgradeToNextV'.  If you don't have that (Which I did not), then you will need to install KB4048614.  This will give you an upgrade checklist to prep your database and modelstore for the upgrade. (This will require going through the standard software update checklist.)

After going through some failed attempts, I narrowed my focus to 2 main paths:

1. Upgrade my company's custom code / data ONLY.
1. This means removing ALL ISV models. This was ok for us as our ISVs have D365 solutions.
2. Upgrade my company's custom data that fits in vanilla AX. 
1. This means removing ALL custom models.

For BOTH options, you must do a full compile, full CIL, and FULL DB sync along with running the new checklist from above.

For option 1, the best idea is to move ALL your customizations into 1 single model.  For some reason, mine were spread out over 4 different models.  You can keep it in different models, however, I think it makes it more difficult.
After attempting a code and data upgrade for option1, we decided to remove ALL non data related customizations. (Classes, forms, etc.) This would leave us with only our Tables, EDTs, etc. 

However, after looking at the amount of work to upgrade this, we decided to give option 2, from above, a try.

All customizations were removed and we began the 'Data upgrade' using the link above.  Our database is pretty big (Over 300 GB). This upgrade process took over 48 hours.  Aside from time, we only ran into a couple of errors in the 'ExecuteScripts' step.  We had to skip 2 steps. To do that, you can follow instructions here. For some reason there were duplicate records in 4 tables so there were some DB sync failures.  They were easily fixed and we were able to get the full data upgrade completed.