Most upgrade projects that I have reviewed had a common theme. The new version was implemented in the old way, that is, all the customization from the previous version were ported on to the new version—as is. This is not the correct way.

As part of the upgrade analysis or planning, you need to spend a good amount of time finding a match for the existing custom features and deprecate the custom features. Most likely, Microsoft may have developed a feature that you had to customize years ago. This is your opportunity to unlock the power of a newer version and maximize your investment in the platform by tearing off customizations. While it may sound like a no brainer, many projects fail to do so, as it needs additional work to migrate the existing data from custom tables to standard ones. A few minor customizations that are in place may be needed on top of a standard feature if the standard one does not completely replace what you have. Hence, shortcuts are taken and all of the old custom code is ported into the newer version.

Also, there are customizations that may not be used anymore. Those need to be identified and removed or left behind as part of the upgrade project. Plan what customization can be replaced or re-implemented with the new version. You should do a detailed fit-gap analysis for new features and see how these features can benefit the business and if there are any gaps. Plan if they can be addressed during or after the upgrade. Fit-gap analysis is also relevant for custom features in the old system—compare these features with any equivalent features in the new version. Identify the gaps and plan to address them as part of the upgrade. Sometimes, such a list of features can be overwhelming and may add significant scope to the upgrade project. 

You also need to make sure that new customizations are done using the new extensibility model. The application suite customization using overlayering will be discontinued after the Spring 2018 release. If you are upgrading to an earlier application version you will be able to do overlayering but this means you cannot uptake new application releases after the application is hard sealed. Refer to Chapter 9, Building Customization for more details about the extension concept and roadmap for sealing the models.