Also is the new disk actually smaller then the old one?
If they are both the same spec/size then it might just be a change of model/revision and the new drive could just have a slightly smaller number of sectors which meant the 3PAR can't recreate the exact number of chunklets as the old one (like maybe one less).
We've seen this a few times, if this is the case then you can tell servicemag to ignore this issue and just continue with the rebuild (with the -partial option).
But yeah if one was 600GB and the other 900GB you'll want to replace it again.