1. Creating a Read-only Replica of MariaDB 

First, create a read-only replica of the MariaDB. 

After that, specify the server name and location to create the replica. When the replica is first created, it will be generated within the same subscription.

For more details, please refer to the following link:

Manage read replicas - Azure portal - Azure Database for MariaDB | Microsoft Learn

2. Subscription Move 

Proceed with moving the subscription of the created replica server.

You can check the created replica under the [Replication] menu in the settings. When you select the replica, you can move its subscription via the [Move] button next to the subscription field.

By clicking this button, you can select the desired [Subscription] and [Resource Group] to proceed.

Please note that during the subscription move of the replica, a lock will be placed on the resource group of the replica server. This means that operations such as adding, updating, or deleting resources in that resource group will not be possible.
(e.g., creating MySQL resources in the target resource group via the Azure portal will not be allowed)

(This does not affect operations within already existing resources, such as insert, update, or delete actions within the DB.)

The original primary DB can still be used without any issues (both read and write operations are possible), and no downtime will occur.

However, it is recommended to perform a test before proceeding.