If you use the "Surveyor" from the SKILL Development Toolbox (Tools -> SKILL Development) you can analyse (tabulate) your SKILL codeand have the resultant file sent to Cadence for automated processing. What you will get back will include a history of changes for each function, showing each version up to the current how the function changed (e.g. there was an addition to the function arguments, or there was an incompatible change to the function arguments etc.) You can specify in the tabulation process whether you want to exclude your own custom functions so that only Cadence functions are considered. You should run this from the most recent version that you have, i.e. IC613. Note that the surveyor exists in IC5141 too, under the name "SKILL Surveyor"
To summarise, there is a two step process with the Surveyor: 1) gather the function names used ("tabulate"), 2) send this file for automated processing.
Of course another approach is to look at the "What's New" sections of the manual, but they are per-release, so you might need to view IC612 and IC613 in order to see what had changed between IC611 and IC613 - the above process is more automated and can help guide you to specific functions that changed, then you could view the documentation for more details.
I hope that this helps to answer your question.