Introduction
If you check Specification Category you will find one quite interesting thread discussing the options to "customize" EHS classic.
This option shows the "highest" option in hierarchy you can use to start the work and generate a complete different EHS Classic "look and feel".
The options to change EHS classic function using the SAP standard are quite large. Some examples are mentioned here:
Customer Exits in SAP EHS MANAGEMENT
Further examples are mentioned here:
EHS Specification Identifier Exit
How to configure VAT of Category COMPOSITION