even if you tell them you are filling it coz you borrowed it from another boc customer, if you swap it and you are not registered for that size/style. you WILL be charged for the rental it happened to me Plus a mate who said I will swap your cylinders,to several people, he said I have an account no problem, he ended up with a BIG PROBLEM, paying 6 rentals but could not stop because he didn't have any cyl's to give back ! they work on the premiss you swapped them you must have them !It will not show up on your account and you may end up being charged for it.
I wouldn't pull any fancy tricks with boc,or any gas suplier, after all,they are going to pass on any costs incurred to those who play fair,& it is effectively a con to work things for a rent free bottle from a suplier who charges rental.
Just my opinion,but I work on play fair,& expect to be treated the same way.