The problem here is the use of storage vMotion. As soon as you utilize storage vMotion, the NetApp sees this as changed blocks (new data), so you will need to re-baseline your snapmirror.
If you want to prevent having to re-baseline, you need to make sure all of the snapshots make it to the new volume. A storage vMotion will not bring the snapshots along to the new volume.
Here is the correct procedure:
SourceVol1 - Current Source Volume on SAS
SourceVol2 - New Source Volume on SATA
DestinationVol1 - Destination Volume
snapmirror update exiting relationship between SourceVol1 and DestinationVol1
snapmirror break relationship between SourceVol1 and DestinationVol1
snapmirror data from SourceVol1 to SourceVol2
Shutdown VMs
snapmirror update from SourceVol1 to SourceVol2
Remove datastore from VMware that lives on SourceVol1
Add datastore to VMware that now lives on SourceVol2
snapmirror resync between SourceVol2 and DestinationVol1 (no baseline needed)
Once you know everything worked as expected, delete SourceVol1
Here is the more official version (should have looked this up before typing all of the above 🙂
https://library.netapp.com/ecmdocs/ECMP1368826/html/GUID-6C850BA4-E522-4F68-841F-D7E273ADE782.html