Here are some more documents produced by ServiceNow, providing guidance about customizing parts of the product:
ServiceNow KB: Instance Customization FAQ and Guidelines (KB0553407)
Some important points from this document:
Since the code is part of the baseline installation and has been customized, the following would occur:
• You would need to maintain that code going forward
• You would be responsible for making sure that functionality still works after an upgrade
ServiceNow will still support you, however, if during a support call it is determined that the customization that was made is the cause of the issue, the support team would advise you to revert back to the code that was part of the baseline installation so that the support team can assist you.
Overwrite a customization during an upgrade
This page is from the product documentation and talks about how to make the upgrade overwrite your customization if a change is introduced in the next upgrade.
Customization Best Practices 101 (repeat 1 of 2)
This is written by a senior solution architect at ServiceNow. It does not address your question directly. It has two labs that provide a hands-on approach to explaining best practices about creating entirely new customizations. It does not talk about altering existing ones, like in your example. However, I have included it here because it has some good general guidelines about considering how to best meet your business requirements.
The main problem with customizing existing UI Pages is that they frequently have references to other scripts and Jelly templates or are referenced by other parts of the platform. Because of the high likelihood of dependencies, it is risky to change out-of-box UI Pages. Like the previous poster mentioned, it is better (if possible) to clone the out-of-box item and leave the original untouched.
