Good luck with that! :
The thing is with Union straps is that the DuPont resin goes all the way
through the interior of the straps and directly connects the ratchet to the length adjuster. This design was to prevent stretching of straps over time, like you get with other brands that only have the strap padding between the 2 connection points.
So you can't stretch them. I would just get another set of SL straps or go with the switching option. If you tell C3 about the issue they should send you a pair without a problem!