SAP provides help for companies who upgrade. You can go through the release notes and check what changes are there.
It also provides guidelines regarding how to go about for upgrade.
To look at schema changes, I suggest:
a)SAP has provided program RPDASC00 to expand the schemas. Run this pgm to fully expand, separately for time eval & payroll schemas, Save each run output in a text file. Do this for the existing system as well as the new one. Then use pgm ConText to compare the two versions of each schema. This will help you to see where changes are made. Then go through the OSS notes and release notes to understand the changes.