Changing research group during a PhD I am currently PhD student, just finished 3rd semester. But it is just 5 months that I defended my proposal, and due to university rules, you are considered an official PhD student just when you defend your proposal. I would like to change my group, due to different reasons, one is financial matters. As my one year financial support from an external fund ( not offered by my current supervisor) is finished and my supervisor also said he has no money! 

I already applied for a new open PhD position (not in the same university but in the same city). The new Professor invited me to his office for interview. Should I tell him that I am already PhD student? What if I say nothing?
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.