I suggest to have a talk with the Qlik representative. He/She should be able to tell you this or at least get you in touch with a (pre-sales) consultant that can help you out. I doubt there would be any cost involved for this.
I am not talking about if you have the license already or not.
You are asking about the risks involved (downtime, processes to change) and what you should do / avoid in this process. I am telling you that the person who is your Qlik representative should be able to give you a clear answer with the question you are having or be able to forward to you a (pre-sales) consultant who can help you out. My guess is Martijn B. .