I've heard of a few techniques for renewing LA batteries, both involve changing the chemistry entirely:
Method A) Dump(and neutralize) the acid, rinse with distilled water until completely clean, then fill with a 10% Epsom Salt solution and begin reconditioning.
Method B) The same as above, but with a 10% Pickling Alum solution.
From what i'm told, Method A creates a differently-formulated LA battery at 2V per cell, and Method B creates a Lead Alkaline battery at 1.5V per cell but a far deeper discharge.
Be warned that I haven't tried any of these methods myself yet (although I know someone running some Epsom Salt batts for several weeks now) so don't take these formulas as gospel...on the other hand, if you have some dying cells you can always play around with them before returning them for a core charge, right?
My neighbor (and stepfather) has a lawnmower battery that's been giving him grief, so I may have a candidate to experiment on in a few weeks-I think i'll just perch vulturelike on his fence for a few weeks...