You can replace a 6x8 speaker with a 6x9, but it won't be easy on most vehicles. The physical dimensions of a 6x8 are more closer to a 5x7 speaker size than they are a 6x9 speaker size. We get it, 6x9's just sound WAY better than 6x8's and are more common. It may be worth it for your vehicle to make this switch. The other factor that makes it tough is the depth of the 6x9 speaker. That is a very common problem area. Rather than cut sheet metal in your car, an adapter ring would be ideal, but they are rare. Check online forum's for your kind of car, someone on there may have used a 3D Printer and might be selling them to other members. An adapter ring would prevent any cutting and at the same time solve the deeper depth needed by 6x9's.