Just a point to remember if this is a production unit with a lot of configuration you can't just add the interfaces to the sd-wan interface if they have a lot of references connected to them (eg. FW rules). So depending on your level of experience with the config file you either have to do a lot of editing on the file then upload it or you will have to remove everything from the interfaces you want to use, add them to sd-wan then redo all the objects and rules etc.
It's a reason why these days i deploy new boxes with sd-wan set up from the begining, even if the customer won't be using it straight away. It gives some useful metrics on the state of their wan link anyway so has value even with just one line.
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