That was my line of thinking when I got mine. My dad has a 26" and 22" barrel for his 870, one for birds and the latter for turkey. I got the 18" version for HD and swap out the 26" when I go hunt.
The one thing you have to make sure of is what length magazine tube the barrel is designed for. The loop on the barrel that connects to the shotgun will be too far away if you get an 18" barrel for a 6 round magazine tube. Make sure you buy one that fits a 4 round tube so you have the ability to swap them.
Oh and remove the plug when you have it loaded with your favorite buckshot for HD.