Garage door weather seal replacement -- Wrap around bracket
You're right about the tension being at a minimum when the door is up. There is tension to hold the door in that horizontal position and that tension can be considerable. What the manufacturer is also worried about is if you remove the bottom roller carriers the door will come crashing down, and they can weigh anywhere from 200 to 800 pounds. You need to support the door in the "up" position, a few "c" clamps and a few small pieces of 1x2's clamped in the roller channels so the door can't move. Do not rely on the garage door opener to hold the door in the "up" position. Then proceed to remove the bolts from the bottom brackets. You should then be able to pry the bracket away from the bottom of the door without actually removing it but freeing up the bottom track so you can install the new seal. Good luck.