This migration is an offline operation .
Below are the setup.
Setup details
vms
vm1: mscluster-n1
vm2: mscluster-n2
We have the vm running on san pRDM and the next is tomigrate it to vsan datastore.
1) Stop the Role in the WSFC.
2) Shutdown the vms participating in the windows cluster.
3) Migrate the firstnode of the cluster to vSAN.
4) Choose change storage only option.
5)select the vSAN storage.
we can see the migration started.
6) Power on node which got migrated.
7) We can see the disk in disk management and cluster running fine.
8) we can see the shared disk on edit setting of vm.
8)From the other node detach the pRDM.
9) Migrate the other node to the vsan.
Once the migration is done go to edit setting of the vm and add existing disk as shown below.
Add the shared disk once you know the vmdk location from the other node.
10)Power on the vm and check the cluster if it is good.
Reference :https://kb.vmware.com/s/article/74786