SQL Server 2012 In-Place Upgrade and Timing of Database Version Change
The database version is updated automatically, when the database is first brought online, during the recovery process.

The database compatibility level is never automatically modified.  If your database is at 100 now, that won't change.

Refer to https://msdn.microsoft.com/en-us/library/bb510680.aspx for database compatibility.

See https://blogs.msdn.microsoft.com/sqlserverstorageengine/2007/04/26/whats-the-difference-between-database-version-and-database-compatibility-level/ for the differences between version and compatibility level.