Don't go lower than 400 grit. Basically, if you wet sand the car using 400 grit, you will remove all the clear coat from the car. Then your existing base coat will be all there is, and it will essentially be like a primer. Depending on how drastic of a change the color is, they may spray another color base coat to mask out your car color before spraying the actual color.

Make sure you wet sand, meaning you dip the sand paper in soapy water every few seconds or so, before it dries out. You'll be able to tell when all the clear is off, but like I said, expect to go over it two or three times since you'll miss spots. If you wet sand with 400 grit, you really can't mess anything up. I wouldn't go lower than 400 personally. Take your time prepping it, and you'll get good results...good luck!

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2004 WRX - WR Blue Pearl
1997 Eclipse GS - Royal Sapphire Pearl