You don´t need to worry about this. DRS / vMotion will not even try to migrate any VM to a host which has not access to the storage system.
If you don´t want to give your old hosts connection to the new storage system you can still use the 5.1 host as a "proxy". First vMotion to it and afterwards performing a svMotion to the new storage system.
In case you have a huge difference between the CPUs, you should also consider if you can may affort a downtime of the VMs for a restart. It wouldn´t be so nice to let the new hosts running with a low EVC baseline, if you are not going to run the old hosts in the same cluster (permanent).
Regards,
Mario