The Cinch system was designed to be used with the Halcyon backplate, we can not guarantee it will function as designed with any other backplate.
Claude, the Cinch system will slide when unweighted and lock off when there is any weight on it. If you can unweight it leaning all the way forward then you can likely slide the straps although it will possibly be a bit awkward. The best way is to find a bit of something to set the tanks on (post, guardrail, picnic bench, dive buddy) and fully unweight the system, OR if the situation allows, cinch it up in the water (as is demonstrated in one of the videos) where you can also fully unweight the system.