Please read the release notes carefully for any changes that may affect you. Also I have a list of my upgrade notes available for download here
Actually there are three "12" versions of QlikView at the moment, and each one comes with its own set of improvements and syntax changes. If you are considering an upgrade to 12.20 (latest & greatest), you should take into account all changes of the intermediate releases. No all changes are documented in the on-line help, so your best chance is as Rob stated to read the different release notes.
These require you special attention:
v12.00: Chapter on "Management of Unsafe Script Execute Commands"
v12.10: Chapter on "Changed behavior in QlikView 12.10 scripts"
v12.20: Chapter on "BNF Script Reload mode"
v12.20: Chapter on "Quotes in Set Analysis expressions"
The last item answers your question on changes in Set Analysis syntax and behavior.