Good luck with that.
We had huge fight with 3par over this. We finally got it done after really hurting our array and running almost down to zero space on a given tier.
First question, are you using AO?
If you are not using AO this should be easy, if you are using AO do NOT let them talk you into doing tunesys to vacate the drives. You have to set the drives to degraded then vacate the data and have the drive re-admitted. We just went through a painful process of this that started back in July!! We finally got the drives balanced in the array, but the data is still not balanced after tuning LDs and the system. Actually have a call tomorrow to see how they can resolve this. Also AO in 3.1.2 is not as good as it was in 3.1.1 when it used System Reporter to do all the work. You loose some of the granular controls you had in 3.1.1 replaced with gimmicky use of the CPG Growth Warning setting. 3par lacks some big picture Enterprise visionaries that understand what the users want and need to do. The technology is awesome, but some of the ways it is implemented still smack of their legacy niche position. HP needs a legacy HP storage person over the 3par dev team.