When upgrading a codebase from Python 2 to Python 3, here are ten steps, things to pay attention to, and pitfalls:

1. Use a version control system (e.g., Git) to track changes and create a separate branch for the upgrade process.

2. Run the 2to3 tool to automatically convert the code to Python 3 syntax, but be aware that it may not catch all necessary changes.

3. Update all print statements to use parentheses, as print is a function in Python 3 (e.g., print("Hello") instead of print "Hello").

4. Check for any use of the raw_input() function, which has been replaced by input() in Python 3.

5. Be aware of changes in integer division behavior. In Python 3, division of integers results in a float, while in Python 2, it performs floor division.

6. Update any code that relies on the xrange() function, as it has been replaced by range() in Python 3.

7. Check for any use of the cmp() function, which has been removed in Python 3. Use key functions with sort(), sorted(), and max()/min() instead.

8. Ensure that any file operations use the appropriate text or binary mode, as the default is now text mode in Python 3.

9. Update any code that uses the itertools.izip() function, as it has been replaced by the built-in zip() function in Python 3.

10. Thoroughly test the upgraded codebase to ensure that all functionality works as expected and that there are no performance regressions.

Pitfalls to watch out for:

• Incompatible third-party libraries that have not been updated for Python 3.
• Subtle changes in the behavior of built-in functions and data types.
• Differences in exception handling and raising syntax.
• Changes in the handling of Unicode and byte strings.