Held hostage by a high performer

Here's the short version of what's going on: I have a health care business and have a high performer receptionist. Patients love her and doctors love her too. She's great and works like a well oiled machine. I recently brought in a business partner who bought in the business and whom I've known for a long time. He turned the business into a cash cow. He has great character, very respectful and trustworthy. She didn't like it, because he wanted things done his way, which isn't adding more work load for her. It's in fact making her job easier. She likes the old ways and is very resistant to change. We suspect she may have paranoia about her losing her job. We reassured her and even offered her a lifetime contract and pretty much a guarantee she won't be fired until she quits or retires. Full benefits package and paid vacation days. But it's not working and we don't get it. They keep fighting and it's affecting the mood in the business. She's been with us since the beginning and we love her and don't want to let go and she carries our business, as the receptionist in a healthcare business is what makes or breaks the business. We could start looking for new receptionists but we are afraid we can't find someone like her. Have you been in this situation before? And what do you suggest we do? UPDATE : Thank you all for your contributions and advice. I did as some of you suggested and sat down with her to discuss this matter and brought up ownership, appreciation and fears of loss of control. She was very candid and disclosed that she felt her role may not be as important as it is going forward. We reassured her that she is still the backbone of our business and pledged nothing will change other than the changes we have made. She said she isn't feeling well and her heart is failing and needs surgery soon and she will take time off and think about retirement. We gave her paid time off until the end of the year and a generous severance package in case she quits and left the door open for her to come back. She was grateful and offered to stay to train more staff until they're ready. We are happy with the results and feel this is resolved. Even if we paid her for the next 10 years to stay at home it wouldn't repay how much she helped us grow the past years.