what you want to do is doable without any problem.
I will have some downtime based on the license type you have, if you have not an option for storage vmotion, you will have to stop VMs in order to copy them to the SAN.
About the backup, once you run alla your VMs from the san, think about a secondary network storage to save the VMs.
Did you already have some backup solution in place?
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Since you are planning to implement SAN (external storgage),
Step 1: Mount the ext. storage into old ESX host as a datastore
Step 2: Perform sVMotion (license permits) or Power off the VMs and perform offline migration
Step 3: Introduce the same ext. storage (datastore) to new ESX hosts
Step 4: Register the VMs and power on them.
You may have problem with backup to external storage method. since the VMX file point to old datastore entries and your VM failed to start. The best option is do a online or offline storage migration.
Worked like a charm...thanks for the response.,