Reverting Compatibility Mode 120 to 100
Is there anything else I can do, or is there a step I missed, to revert back to the old Cardinality Estimator?


In SQL Server 2014, reverting to OLD CE can be done in 2 ways


Setting the database compatibility mode to less than 120.
(or) Enabling trace flag 9481 globally or at query level using option querytraceon


Now that you have reverted to old CE, your next steps in troubleshooting are finding out/isolating queries that are poorly performing - using XEvents or server side trace.

You should tune those queries. 

My Suggestions :


Dont revert to old CE, instead isolate poor performing queries and tune them. Use option (querytraceon 9481) as query hint if you are not able to tune and want specific queries to use the OLD CE.
Since you are on SQL Server 2014 + SP1, you should enable trace flag 4199.


Also refer to 


Recommended updates and configuration options for SQL Server 2012 and SQL Server 2014 with high-performance workloads