It sounds like what you are not doing is removing divisional command.
Remove divisional command which returns all units to a pool.
then you should be ready to create a corps by selecting the create a corps command.
Once the corps is created, the corps command can no longer attach units, that is when you know you have succeeded.
Then other division commanders can attached the released units.
So just tested, when I remove division command from McLeanard, the option to create a corps is immediately available. He has all the previous units still attached, but not in his unit.
Play around some more. The learning curve is steep at first, but it will become automatic soon.