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The GPD rating of a membrane has nothing to do with how much the membrane removes. RO membranes have a "rejection rate," usually 90% or 98%. Of course, the higher the rejection rate, the more is removed from the water. Naturally, we want to get the 98+ rejection rate membranes, as they produce purer water. And if you have a DI stage, the DI resin will last longer with a 98% membrane since it will have less "work" to do on the water passing through it.
It's important to note that 100gpd membranes are usually just 90% rejection, which really isn't ideal for reef tanks. I'm not sure what rejection rate your average 24gpd membrane is, but it's probably 98% rejection. I'd Google it just to be sure.
As far as will it fit in your housing, I don't know. If it does fit, though, and you want to use the 24gpd membrane, you'll need to buy a new flow restrictor. You want a 4:1 ration of waste water to purified water, as in for every five gallons coming out of your unit, 4gal should be waste water and 1gal should be purified water coming out of the membrane. Waste water washes most of the particles off of the RO membrane as they're being filtered out, and then the waste water carries them out of the membrane so the membrane doesn't get clogged. So, waste water is what allows your membrane to last 2+ years. Your flow restrictor for a 50gpd membrane will not produce the proper 4:1 ratio using a 24gpd membrane, so you'd have to change the flow restrictor out for one designed for a 24gpd membrane.
Finally, how do you know your membrane needs to be replaced? I'd hate to see you spend the money on a new membrane unless you're absolutely sure your current one is bad.
-Joe
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75gal RR. Custom bi-level sump with built-in RO/DI top-off water tank. GS1 cone skimmer. Tunze Osmolator top-off system. Aquacontroller Jr. w/ DC8. TEK 48" 6-bulb T5 light fixture. Empty bank account... 
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