I suppose it would help if I finished the directions. After you put the general you want to command the new army in the renamed HQ stack, You have to make him army commander like normal. Every time I have done it you still are able to Attach Corps to the renamed army. So for my previous example you ...

You can however rename the Army Stack as long as it is just an empty HQ.
For example, say you want to make a Army of the Atlantic or something, take an army HQ and rename its stack and then put the leader you want into that stack. If you do it the other way it does not work.