Will I lose my GI Bill kicker as a reservist if I do ROTC and commission as an officer before my reserve contract is up?

Yes.
It is a whole new contract with different stipulations and bonus addendums.

There are two answers to your question. Once you commission, you start a new service obligation and will not receive a GI bill kicker as an officer. However, you may still have Reserve GI Bill eligibility depending on how contracted and into which component you commissioned. Before commissioning, you may still be eligible for a kicker as an SMP cadet. However, a kicker you received based on your MOS may go away. There are certain scholarships that cause you to lose your GI bill and some that don't. Rule of thumb is that if you lose GI bill eligibility, you lose the kicker. But each person is different.

Once you commission you will be on a new contract and there will not be a MGIB-kicker. However, anything you used prior to does not have to be paid back. Not sure if that answers your question.