Hello Thanks for joining the site and posting your question. This is an interesting one I don't think I've ever encountered. In general, does your French horn rest on the area where your current pacemaker is and where it is to be replaced? I wouldn't think so. And I wouldn't think this will impact your ability to play. However, recovery from the replacement can take a few days to a month or so. You may have bruising, pain, and often you should take a few days off of work to recover. It's a very quick surgery though. I think 6 days is potentially a little too soon but everyone is different. What does your doctor say? https://www.mayoclinic.org/tests-procedures/pacemaker/about/pac-20384689
Let us know how it goes and French horn is one of my favorite instruments!