When I installed my slide toppers they had cotter pins through the roller and end caps to prevent it from loosing the spring tension.
I would think that you should extend the slide some (6-8 inches) and insert some pins then retract the slide the rest of the way. This should relieve the tension on the fabric without loosing the tension on the spring.
Maybe if you give yourself more than 6-8 inches you would be able to remove the brackets and set the roller on the roof and have it out of the way for the rest of the project without removing the whole thing.
Mike
2006 Chevy 2500 4X4 crew cab Gasser
2013 Dutchmen Denali 311 BHS (Love the trailer HATE the manufacture!)