They’ve got it right, but I’ll add a little to it as I was speaking with DAS today about this.
If you change the global order configuration (setup menu, order template), you have to make sure to check the “use global” box when you right click the montage and go to order template.
Either way you do it, if you have an account chosen in the order template, it will always default to that account when you click a symbol, assuming you have the account set for the exchange on which that symbol is traded. So if you set the default as your U account from IB, then you change it in the montage to trade your demo account, it will switch back to the U account (the default) when you click another symbol.
To avoid this, don’t choose an account in the template. Leave it blank. When you choose it in the montage, it will stay that way until you change it again in the montage.