Well, I went ahead and did it.
Moved all the data from 6 2TB drives to 3 4TB drives, all done in my separate "preclear box".
Re-organized my main array, did the complete parity sync, then a complete parity check afterwards.
Completed fine with 0 errors. All data intact.
Seems that if you are removing drives, there will have to be a new parity sync, and you will be at risk if one of the drives you are not replacing happens to fail during that sync. So whether you do it one at a time or all at once, there will be some risk at some point. Accomplished it all much faster by copying outside of the array.
What I was thinking that I could have done is to do the parity sync on a new drive. Since I keep one 4TB as a hot spare, I could have rotated that one with my original parity disk. In case of failure, replace the original parity drive and all the original drives and rebuild from there.
Sorry about the thread hijack, but since this is the lounge and we are talking about uses for a separate preclear box, I figure it is ok.