Hi Kadkhah,
Shorter lines are a good experience, if you have the skills and experience to use them. Shorter lines do the following:-
1. Make your kite a little faster through the air
2. Make your kite react more quickly
3. Shorten the distance from one edge of the window to the other
4. Make your kite sit higher in the neutral (depowering your kite)
5. Shift your useable wind range up about 5 knots per 5m
A 12m will fly badly on anything less than about 20m lines unless you are using it for wave riding on a surfboard and need the speed in moderate winds.
The power becomes very "on and off" and may really annoy you in gusty winds. Wave riding is better because speed and timing are improved which are essential to wave riding.
Kite loops are more radical because the kite does a normal sized arc in a loop turn, but has to do it in a greatly reduced wind window thus forcing your kite down lower to the water essentially allowing you to get as high and often higher than the kite!

Good fun for the brave!
The other guys are right, buy a smaller kite, like a 9m or an 8m for winter and enjoy the speed, it will quikly become your favourite kite.
If you do decide to get shorter lines, buy a second bar with 20m lines and get a 5m extension set too which will give you far greater flexibility to tune and change. Don't cut down your current normal length set, you'll spew once you get to lighter winds with the hugely decreased bottom end power.

Good winds,