Changing research group during a PhD
It is crucial to tell the truth.


For your current supervisor, you will need to let them know that you are interviewing with the new Professor, and explain why you are doing this. This potentially could work to your benefit as your current supervisor may offer to be a reference and offer assistance to you going forward.
For the new Professor, let them know upfront that you are already a PhD student and the reasons why you are moving.


Most of all, as has been said in the comments, be honest - because if you are not honest, it can (and probably will) blow up in your face, potentially generating distrust from the old and new supervisors/Professors.

In your comments to the question, you mentioned that you are not sure as to whether or not you are going to leave your current position - before you progress any further, make sure you are certain in what you are doing (at the very least to be sure you're not wasting anyone's time).

An acquaintance of mine did something very similar, and was caught out a few months later - he had to overcome significant suspicion about the reasons for his move, as he had not explained it to both parties.