The 875 is for kids who need that strength of boots; they may be working on singles or doubles or triples
There's no actual correlation between "jumps landed" and "boots needed" otherwise it would be a lot easier to buy them!
My one kid is in a boot stiffer than that one, but, only lands the occasional double - however, because of height, weight, stroking, leg length, etc, she needs the additional strength of the 1500. When she went into the 875, she was landing only singles - but, she broke them in in 4 weeks, which was a good indication of a good fit.
My other kid who is 150 pounds and 5'10" tall is landing triples in a boot that is not as stiff as the 875. And that's with a new reboot to a stiffer boot recently.
A good test is pretty basic: Put the boots on the kid. If they cannot bend them to the point where the knee goes beyond the toe, then, they are overbooted. If your kid had a long time to break in the 435 ... unless something has radically changed ... she would likely be way overbooted in the 875.
BTW: Riedells when fitted should be really really snug in the foot. In about 3/4 weeks, the internal 'foam' will compress and they will fit perfectly. The biggest mistake made in the Riedells is to get them so they are "comfortable" as they will then end up too loose once hte internal "melting of the lining" happens.