Good question!

I think importantly, you should be confident in your ability to learn quickly and express that confidence. Also, express excitement about the features of Python that are simpler than Java, but be thankful that your background in Java will give you a great starting point.

Basically I think the interview process is really about how you position yourself, and if you tell the right story, you'll do well.

Otherwise, I'd spend some time messing around with Python in a REPL environment. This is where the magic happens in Python.

Express your passion for Python and show'em really why you want to become Python Guru.
Be confident.
Read/Learn/Code/Help Others.
Have a short and long term goals planned.
Come back and update us on progress in few weeks.

Whenever we are into a task, we should be positive, goal maker, and tackle your ideas and a strong knowledge about python might help you. I would like to suggest you one more thing, prepare for the worst will definitely reflect in a better result

It depends on your future employer. My experience is, that showing the ability to learn and your social skills (or better: understanding business) are more important than your experience with the language. It is from my perspective easier to get people on track with the language if they are able to pair, than having a professional programmer who knows the language, but doesn't understand what business requirements are and when what solution is a fit and when and how to make appropriate tradeoffs.

I made the switch from .Net to Java, to Javascript, to Python and a bit of Go. People don't bring me mainlyinto projects because I have previous knowledge - which sometimes is also the case ;) - but because they know I am a good building block to work with.

Currently I am doing GIS which upon recently I had no clue about. But I am learning while doing the software.