Knee replacement surgery will give both of you a new lease of life. Providing you apply yourself to your rehab with the same gusto you do your MA, you will be able to come back to pretty much full activity. Doctors advise on moderating activity with a new joint so that it lasts, and you dont have to go through the same surgery all over again when older. As a sports therapist I have to say that by keeping the legs active and the leg muscles strong you will be looking after your knee's future far better than the softly softly approach.
Also, if you stop your current activity completely, your legs will weaken, and so your knees will hurt more. Merely cut back on high impact work (running, skipping, and sparring) but use bikes and cross trainers instead as a pre op alternative.
Also, avoid the leg extension station when weight training, and do squats instead. Only within a comfortable range of motion, but they are the best thing to maintain the structure and strength of your knee joints.
As to what to expect afterwards, hulk Hogan had double knee replacements and a new hip, this allowed him to come out of retirement and wrestle again. No matter your thoughts on this activity, it does display the athleticism and roughness that the replacements will withstand if you ask them to.