Just to come back to this:
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Hello Elliot, finally got an answer for you.
SmartNet is 100% needed as confirmed by the BU. If you were to convert a license that had DNA License for 3 years and SmartNet for 2 years, you would be credited with a 2 Year Meraki licenses
So when converting from DNA -> Meraki, whatever you have the least off is the duration of the Meraki license (as you stated above). If that was to happen, we would strongly urge the customer to increase the lesser part of the DNA side to maximise the Meraki license."