1. No. Environments are independent. Each environment is created under an Azure AD tenant, and its resources can only be accessed by users within that tenant.
When you create an app in an environment, that app is only permitted to connect to the data sources that are also deployed in that same environment, including connections, gateways, flows, and Common Data Service databases. For example, let’s consider a scenario where you have created two environments named ‘Test’ and ‘Dev’ and created a Common Data Service database in each of the environments. If you create an app in the ‘Test’ environment, it will only be permitted to connect to the ‘Test’ database, it won't be able to connect to the ‘Dev’ database.
According to the documentation:
If you intend to upgrade your previous version CDS database, you muststart the first step in the upgrade process before March 15, 2019.
If you do not start Step 1 before March 15, 2019, we will disconnect your database and store your data for 30 days before deleting it. If you wish to reconnect a database, you must contact support before March 29, 2019.
Important: If people take no action, we will expect that they are not interested in upgrading and their databases will be disconnected on March 15, 2019 and deleted after 30 days.
Since you still didn't upgrade, your database should have already been diconnected. Please contact support now as soon as possible here:
I proceeded to step 2 with no Resources so I raised a ticket with Microsoft.
Microsoft explained to me that in that case, there were no Apps or Flows using the (old) CDS.
So, I proceeded to Step 3 and the upgrade went successfull, with only 1 warning:
XrmSystemUserForAadPrincipalMissing: We’ve finished migrating your approvals. Please note approval records where approvers are no longer in the tenant or are disabled in the tenant were not migrated and won’t be listed under Approvals History.