I would not go smaller than a 9.

Shortening the lines is also an option, but I find the kite flys a bit too jerky for my personal preference.

If possible, buy a 10m in the same cut (model/brand) as your current 12m.

When the wind strenghens and you need to downsize, it will 'feel' right.

i was using 15m lines for a while so that I could extend the wind range of my 11m.
I found that it didnt really increase the range as such, rather it gave you more control in high winds. The kite flies to the edge of the window a lot faster so when it is really cranking, you can handle it a lot easier. The power strokes were shorter, the jumps were not as high and you there wasn't the same hangtime although I felt there was more of an explosive launch when you sent the kite. Wakestyle riding was more or less the same.
Nonetheless a lot of fun! Having your kite right in front of your eyes is awesome. Kiteloops are full on. Waves are different (you need high wind but that wouldnt be the case if youre just using it in higher winds anyway). Less time sorting out your lines!
Big down side is swapping back to longer lines and/or using a larger kite on 27m lines. Also, it is a lot harder to get the 5th line sorted out especially if it is kite specific (i.e. is a different length to the other 4 lines). I would suggest having a spare bar with the lines ready to go. Remember also that it will affect the kites low end.
If you want to start a quiver, get an 8m so that there is not too much overlap.