One plate is lead, one plate is lead dioxide. Hand-waving a big: sulfuric acid dissociates in water, making a lot of hydrogen sulfate ions. The lead plate reacts with the hydrogen sulfate to make lead sulfate, one hydrogen, and two electrons. The lead oxide plate takes a hydrogen sulfate, three hydrogens, and one electron to (also) make lead sulfate. This means there are three lead reactions required to make sufficient hydrogens for the lead oxide reaction, but the lead oxide reaction only consumes one of the six electrons generated. That is, the process releases 5 electrons.
The process also generates a considerable amount of lead sulfate. This forms as a powdery coating over all the plates in the battery. Interesting tidbit: any mechanical shock (bumps in the road, etc.) can knock that powdery coating off. As the coating is no longer attached to a particular plate, it's not going to get "energized" when the charge reaction happens, and thus will not convert from lead sulfate back to lead or lead oxide. Eventually, enough of this powder accumulates on the bottom of the battery and will short two (or more) plates, resulting in a higher self-discharge rate. Eventually, the self-discharge rate gets high enough that the battery won't "hold a charge" and then it gets replaced.
So, anyways, if all you do is pump out the electrolyte, then you haven't done anything to address the fact that the lead plates in the battery have corroded away (to form lead sulfate). Replacing just the electrolyte doesn't charge a battery. You would have to replace the electrolyte AND the lead plates.
But, if you take a dead battery, and then replace the electrolyte and charge plates, then it's a new battery. The easier thing to do than cracking open the battery and replacing all the internals would be to just have an exchange program, like propane tank exchanges in the US. Users are charged a fee which equates to the energy (gas, or in this example electricity) added to the "vessel" as well as a small service fee to clean/inspect/recondition the "vessel."
Personally, I think that's the way of the future for electric vehicles. It's just too hard to provide the electric power required to charge a battery at anywhere close to the rate gas vehicles refuel, which "charge" at a rate of megawatts - check energy density of gasoline and multiply it by 10 gallons per minute.