You do not have to replace your entire network to adopt SDN.
Overlay model: SDN controls virtual networks on top of existing infrastructure. Physical switches unchanged. Lowest risk. Used by VMware NSX.
Underlay model: SDN controls physical switches directly via OpenFlow. Requires compatible hardware. Maximum control.
Hybrid model: Traditional protocols handle base connectivity. SDN handles specific use cases. Common in brownfield environments.
Vendor-specific: Cisco ACI, Arista CloudVision, Juniper Contrail. Tight integration. Vendor lock-in trade-off.
Start with overlay for quick wins.