Windchill 11.0 to Windchill 12.0

Hello,

I am to upgrade customization code for Windchill 12.0 from Windchill 11.0. I have my source code and all Windchill 12.0 jars, but since many APIs have become deprecated and removed from 12.0, please let me know how to start. 

Thanks in advance.

Regards,

Sha