> 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#...