I just went from 4 to 6 liters and like you, my concentrator and O2 machine stopped at 5L.
The medical co. switched out my O2 machine with one that goes up to 10 and supplied me with high flow green tubing for around the house.
Now, going outside of the house is a little tricky... They are telling me to use the full flow regulator for my travel tanks, and they gave me this tubing that has a suction bag on it that holds the oxygen as it's coming out of the small tank and 'conserves' it and as you breathe in, the suction releases and delivers you the oxygen....they said it's supposed to last just as long as using the concentrator, but I don't think it does. I've gone as far as using that tubing with the bigger tanks that I have to wheel around.
It's a pain, but you work it out =) Don't fret about it until the time comes!