Changing the Parametization option in SQL Server 2012 from Simple to Forced Does changing the Parametization option in SQL Server 2012 from Simple to Forced clear the cache; or should DBCC FREEPROCCACHE be run after the change?
It frees the plan cache for the database that you change the option for, and nothing else.

On my server, I run these queries:

USE StackOverflow;

SELECT TOP 1000 * FROM dbo.Users AS u WHERE u.Reputation = 2;

USE StackOverflow2010;

SELECT TOP 1000 * FROM dbo.Users AS u WHERE u.Reputation = 1;


They I use sp_BlitzCache to look at my plan cache:



After that, change one database to use Forced Parameterization:

ALTER DATABASE StackOverflow SET PARAMETERIZATION FORCED


Give sp_BlitzCache another run:



Only the plan from StackOverflow2010 remains.

Full disclosure: I contribute a lot of mostly working code to the open source sp_BlitzCache stored procedure.