I was having this same problem and couldn't find the answer anywhere but I figured it out.  Let me walk you through it. You have a parent account, then you have subaccounts and you may also have subaccounts of the subaccounts, like me.

1.Your going to start with the lowest of the hierarchy (the subaccounts of the subaccounts).  Go into edit account and remove the check mark, no longer making them a subaccount of the subaccount, and save.  This will cause them to move on your chart list.  They will move depending on how you have them either by numbered account or alpha.

2. Then your going to go to your subaccounts and remove the check mark, no longer making them a subaccount of the parent account, and save.

3. Now, you can change the type and save the parent account.

4. Since, you removed you subaccount from the parent account, you will have to go back and find them on your chart list and check mark subaccount of the original parent account and save.  You will have to do the same for the subaccount to subaccount, if you have any.

I hope this works for you.  I was wrecking my brain trying to figure it out.