If the purpose is to apply stimulus intense enough to trigger muscle growth, then the "high" rep vs. "low" rep becomes irrelevant, in a sense. If you are looking to give your joints a break, then lowering weight would seem like an obvious. However, that does not automatically mean more reps. Secret word is "tempo". Slow down your rep quite a bit, you will be forced to lower the weight and the decrease in momentum brings down the impact on your joints caused by quickly going from eccentric to concentric. High reps and slow reps are similar in that they both require less weight, and they will both increase time under tension, but they differ in the sense that slowing down your reps eliminates the impact mentioned above. It is possible to up the reps with lower weight but still apply much impact to one's joints, it sounds like that is not what you are looking to do (your joints are hurting).
Therefore my advice is; lower weight, drastically slow down the speed, or tempo, and you'll be able to recruit the same amount of muscle fibers, keep rep count low, and eliminate impact on the joints.
That's my advice and I'm sure some would disagree but it is not something I just pulled out of my ass and is supported by the science. For the sake of keeping this short I didn't go into all of that (science stuff).