2 reasons. It will change the profile to shorten BC, or lengthen Z and shorten B, or lengthen Z and lengthen C. Second it will limit depower, which if user doesn't know how to use the bar to make kite more stable may be helpful for them. The negative is it limits depower, so lost range (low and topend) and speed. It also changes the profile in a weird way, which shortens B more than C. But honestly that may be the profile change you need as it is usually the more stable type. I just don't approve of the depower limit change as it is unnecessary, because you can always limit depower with the bar.
If you want to have the same profile as lengthening A but with the same B limit. You would lengthen Z and C. If you want to increase B limit for more or original depower then you also lengthen B, but then also C and Z more to maintain the profile.
If you want help converting an A change to BCZ, just let me know.
There is a post above about sonic fr 11m with the same issue where shortening b, c, and z solved it (same effect as lengthening A main).
I am just going to correct you on what was posted
Now I have ended up at: a1(0), b1(80mm shorter), c1(60mm shorter), br1(150mm longer)
This says B shortened, C shortened, BUT Z Lengthened. This is not the same as lengthening A
But any way flysurfer must release instruction for such cases because its not normal that after one year such expensive kite is disbalances regarding shrining and people must spend hours of speaking and experimenting on how to tune it !
They do give general advice which I don't completely agree with. Specific advice is too hard as production and use variables make things different.
The main problem is how they go about adjusting. If they just had a Z adjustment, it would be so much simpler to explain, understand and do.