Okay here's the actual peice you want to read in Darren's link.
The actual documentation is misleading because it says you need "Per Device SD-WAN+ licenses" which are only available in 1,3,5 year terms.
I have a 10 year SDWAN license which is clearly not a per device license. The text below does not state it needs to be a per device license however.
Mixed-license environments
The SD-WAN Plus and Advanced Security licenses can be selected on a per-device basis for a mixed-license deployment. In this way, some sites could have MXs with full SD-WAN Plus license functionality and remaining sites with Advanced Security license features. Enterprise licenses can only be applied organization-wide.