Hi,
Parallel deployment is a common migration scenario from traditional to SD-Access.
If you look at the SD-Access design guide, you'll see that the Fusion device is a separate entity that allows connectivity to other L3 networks as well as shared services and route-leaking between VNs if required.
So, you end up with a device(s) typically with collocated Border and Control Plane functions, then separate device(s) that run as fusions.
Hope that helps.
Regards.