If they have not been used for couple of years and not kept charged they are likely bad from sulfide crystal formation.
As to changing electrolyte, fully charge the battery before evalating or changing the electrolyte.
Discharge converts lead + sulfuric acid into lead-sulfate and water, charging reverses this. Charging is complete when you converter all the lead-sufate back to lead and restore the concentration of sulfuric acid (normally about 35% at full charge) The recharging process needs to happen within a few weeks to maximum of about 6 weeks or the fuzzy soft lead-sulfate crystalizes on the plates. Once it crystalizes it is locked up and will not reconvert during recharge.
As to hydrometers they are selling in most auto parts stores, if they are half moon shaped with little black plastic pointer, don't waste your money. Get a good tube-float design, about $20 to $25.
% State Of Charge Specific Gravity Corrected 77 F Open Circuit 12-Volt
- 1.277 12.72
- 1.258 12.60
- 1.238 12.48
- 1.227 12.42
- 1.217 12.36
- 1.195 12.24
- 1.172 12.12
- 1.148 12.00
- 1.124 11.88
- 1.111 11.76
- 1.098 11.70
- 1.073 11.58
- 1.048 11.46