Once this new database compatibility default goes into effect, if you still wish to use database compatibility level 130 (or lower), please follow the instructions detailed here: View or Change the Compatibility Level of a Database. For example, you may wish to ensure that new databases created in Azure SQL Database use the same compatibility level as other databases in Azure SQL Database to ensure consistent query optimization behavior across development, QA and production versions of your databases. We recommend that database configuration scripts explicitly designate COMPATIBILITY_LEVEL rather than rely on the defaults, in order to ensure consistent application behavior.
For new databases supporting new applications, we recommend using the latest compatibility level (140). For pre-existing databases running at lower compatibility levels, the recommended workflow for upgrading the query processor to a higher compatibility level is detailed in the article, Change the Database Compatibility Mode and Use the Query Store. Note that this article refers to compatibility level 130 and SQL Server, but the same methodology applies for moves to 140 for SQL Server and Azure SQL DB.
It’s good to hear, and as Joe mentions, you have the ability to move back down to 130 if you need it.