Yes, it is reversible.

You can use a lead acid battery charger to recondition your battery to certain degrees. The charger should have 4 modes. One of them is called "equalization" charging which is to break down hard sulfate crystals. Your battery lost capacity becasue the plates in your battery have lead sulfate (PbSO₄) build-up which increase its resistance. If you could remove the lead sulfate from the plates, your battery would become a new one. Some people can open up lead acid battery, and clean positive and negative plates, and add new acid. But that is a complicated process, and is not for average Joe. You can also open caps on top of your battery and check acid level. If the level is low, adding more distill water will generally bring up capacity of your battery.

Best regards,

Alex