> I need to wait until dashboard enters grace period (30days) and the ask Meraki support to convert it to subscription model?
You wait until the co-term licence expires, and then claim your subscription code. It will then convert to subscription licencing. There is no need to contact support.
>I will then have only 30 days to buy&claim all subscriptions
You can buy a subscription and request it to start at a future date (this is what I usually do).
https://documentation.meraki.com/General_Administration/Licensing/Subscription_-_Licensing_Overview#...
>Maybe I can buy/claim them earlier
You can buy them earlier - but you can not claim then till your co-term licence expires.
>It would be much easier if I could buy all subscriptions centrally
A subscription can only be used for a single org. You can then bind it to one or more networks. You can have multiple subscriptions within a single org.
https://documentation.meraki.com/General_Administration/Licensing/Subscription_-_Licensing_Overview#...