You don't have to break the tire seal on the rim. Cut the old stem off and jam it back into the rim. The use some liquid soap and a screw driver to work the new stem in from the outside. Not easy but can be done. Sometimes easier than breaking the tire seal.
Just did this an breaking the seal is not a big deal. You don't even have to take the tire of the rim to do it either. I tried it both ways, and letting all of the air out of the tire, cutting the stem, breaking the seal, and putting in a new valve was easier to me.
