We are moving from latin1 to UTF-8 and have 100k lines of python code.
Plus I'm new in python (ha-ha-ha!).
I already know that str() function fails when receiving Unicode so we should use unicode() instead of it with almost the same effect.
What are the other "dangerous" places of code?
Are there any basic guidelines/algorithms for moving to UTF-8? Can it be written an automatic 'code transformer'?
Can it be written an automatic 'code transformer'? =)

No. str and unicode are two different types which have different purposes. You should not attempt to replace every occurrence of a byte string with a Unicode string, neither in Python 2 nor Python 3.
Continue to use byte strings for binary data. In particular anything you're writing to a file or network socket is bytes. And use Unicode strings for user-facing text.
In between there is a grey area of internal ASCII-character strings which could equally be bytes or Unicode. In Python 2 these are typically bytes, in Python 3 typically Unicode. In you are happy to limit your code to Python 2.6+, you can mark your definitely-bytes strings as b'' and bytes, your definitely-characters strings as u'' and unicode, and use '' and str for the “whatever the default type of string is” strings.
How to move a python 2.6 project to UTF-8?