The issue in the past to do this was a reliable means of assessing impact and testing change. Somehow you need to show the business process you had before would not change (or change in a small way) when you replaced extension with base. You needed the confidence that your business can operate without that need for the extension.
This is where the Oracle Utilities Testing Accelerator (UTA) comes into play. The key concept in the tool is the modelling of business processes using flows to represent the business process and the components, base and custom, forming the basis of that process. When using UTA, you can model your business process (as is or how you want it to be) in the tool and experiment with configuration and code changes to remove extensions and replace them with base code, reflecting those changes as necessary. You can compare results of tests at various levels to decide the go/no-go decisions to migrate off an extension.
There are three outcomes with using UTA in this fashion:
The advantage of using UTA is that can set the pace of your migration to match your risk tolerance and your deployment plans.
For more information about the Oracle Utilities Testing Accelerator, refer to the Oracle Utilities Testing Accelerator Overview (Doc Id: 2014163.1) available from My Oracle Support. This contains an overview. FAQ and datasheet on the product.