I had to do the same about a month ago, I ended up contacting EMC support to assist as I could not find the correct procedures in Solve or the support site.

What's in the admin documentation doesn't work, hence my post ...

What I ended up doing, that EMC support couldn't find:

- create the new metavolumes on the new storage

- remove one old copy of the metavolume (detach-mirror)

- attch the new copy (attach mirror) and let it sync

- redo with the second copy

- reconfigure the daily metadata backup to use new storage volumes on the new storage

- retire old volumes

hope this helps