I have a relative in sports who has a similarly timid coach and it creates tremendous problems for everyone on the team. Nobody unwilling to take important actions out of fear of hurting some people's feelings should be in a leadership position. Your feelings matter too and are neglected when your coach says they want you in a bad position throughout the season because they're unwilling to do their job.

Unfortunately, you and this other debater are going to have to do your coach's job for them by directly talking to the partners involved, saying that you'd prefer to switch, and getting their agreement to switch. Be nice, obviously, but don't be fake. Possibly, they'll look forward to switching. Mismatches aren't fun, especially when they're mismatches of work ethic.