What happens if you just loosen the shoulder straps (and possibly tighten the waist/hip strap)?
I don't really like to wear a hydration pack on the bici, just because when it's hot enough to need one, it's too hot to have something on my back - but on hot days, I really, really need the extra water. I'm the queen of sweat

and it can be 30 miles or longer between water stops.
I basically have three settings for the straps: short for walking/outdoor events; low on the hipbones for cycling; and blow it all out when it has to go over a motorcycle jacket and armor.