Sure you can change it
God how incapable these service hotlines are. Really makes you think what they are doing all day.
That Custom Programm button is something that every LG FL of your washers generation should have. So it is not a complicated feature, and it worked the same ever since it got introduced.
This LG has 2 ways to run preset programms:
1) If you turn on the machine and press the start button without touching anything else, the machine will automaticly start the Normal cycle with its presets (which should be a warm wash, cold rinse, high spin speed and medium soil on yours).
2) The custom programm button allows you to store any selectable programm combined with any selectable options or other changes to reselect by pressing that button. Just select the desired cycle, options and modifiers and hold down the custom programm button until you hear an audible signal and the light on the button turns on.
The cycle is now saved until you overwrite it. That means (as far as I know) that even if you select the custom programm button and the decide to - for example - change the wash temp, the cycle stored under ther custom programm button will not be changed.
I could be wrong about the last bit of info there though and it is always a good idea to double check your settings to prevent possible damage to the garments.