First of all, you may alter line sets. When you do, the nitrogen charge is released into the atmosphere.
You can then pressure the lines with a tracing gas in order to check if there are leaks in the connections you've made.
After the test is complete, you have to evacuate the line set before opening the valves to start the system.
Knowing this, we can say that line sets, when altered, may NOT be treated as self-contained systems of their own is a FALSE statement.