It seems to me that the key thing here is to unclaim the license you claimed as a renewal, as per the instructions in the document you referenced. When you claim a renewal in a co-term Organization, you must include a license for every device that you want to carry on working afterwards (i.e., including your MX67's) Ajit is right that you must have the same license level for all MXs in an Organization. If you want Advanced Security, then once you have unclaimed the renewal you should ask Support to convert the Organization to Advanced Security (you will see the license time, til renewal shortens - as Adv Sec uses MX license time at twice the rate of Enterprise) You will then be able to claim the license with the MX250 Adv Sec into your Adv Sec Organization. With all your MXs in the same Organization, setting up (Auto)VPN between them will be so much easier and more powerful than setting up non-Meraki VPNs between the MXs in different Organizations.
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