There are a few ways you can go about doing this, but it depends on your license level, your hardware in these hosts, your external infrastructure, and other factors.
- Shared storage migration. You present the same shared storage accessible to your "new" hosts to the "old" hosts. You then storage vMotion (svMotion) the "old" VMs onto this shared storage. Once complete, the "old" VMs are shutdown from the source and unregistered with the process reversed on the "new" host side. This requires you have appropriate licenses on the "old" hosts (which you didn't state), shared storage, and ability to connect to that shared storage from both sides.
- Backup and restore. Using one of a variety of tools, you backup the "old" VMs from the source and restore them to the "new" side through your vCenter Server.
- Export VMs. You export existing "old" VMs as OVF/OVA appliances from the source; store them at some location; then import them into the "new" vCenter Server side.
- Replicate VMs. Using one of a variety of tools, you perform a replication of VMs from source to destination. Once the appropriate window has been reached after the data is replicated, you perform a final cut-over and fail over to the replicated VMs on the "new" side.
Each step has its own procedure, requirements, and caveats, so without knowing more about your infrastructure it's impossible to say which one works "best" for you.
