If you choose to change mos before fulfilling your contract and without receiving your entire bonus, how much do you have to pay back?

If you reclass to a bonus granting MOS you should be fine. Reach out to your Career Counsellor and, SSG (Join to see) said, check your contract. Everyone's is worded differently so the advice you receive may not be entirely accurate.

It will be pro-rated. based on what they paid you and how much time you have left on that MOS contract.

My unit asked that I reclass to an MOS that they needed help in. I did not have a bonus but when I reclassed during that enlistment I lost my student repayment bonus. So read the contract SSG Hurlock said. I would have never have thought that I would lose my SLRP.