Generally, would you use the same length lines for a 9m and a 12m?
Whats the purpose of changing your line length and when would it be considered?
Normally you would use the same line length for a 9 and a 12m2.
The behaviour changes dramatically, when you change line length.
Often many prefer somewhat shorter lines in waves, as the "reaction" time is shorter and you can change direction (redirect the kite) very easy and fast.
For low winds - longer lines are preferred.
You get a longer sweep thus you can get more power "on demand" so to speak, and it will generally be much easier to work through lulls now, with minimum work.
For general riding TT and freestyle, it is personal preference whether you want a bit shorter or longer lines.
Too short, and the kite becomes too much on/off and dont feel right
Too long, and the kite will get a "saggy" delayed feel, and dont feel right