I currently work PT at O'Reilly's just for the discounts and pocket change. The 'Free Replacement" of a Battery is a true statement to an extent, yes you get a free battery in the event that your battery takes a crap (for example) in the 42 month warranty period.
However, your 42 month warranty DOES NOT start over! Your warranty is only applicable to the original purchase date.
So if you bought a 42 month battery on Jan 1, 2022 and it took a crap on Jan 1, 2025 then you get a 'Free Replacement' battery with a 6 month warranty!
O'Reilly's, AutoZone, NAPA, Advance all use the EXACT TERMS for Battery warranty. Yes some stores will try and work with you and your battery needs, but don't expect or demand it! I know that O'Reilly and Autozone don't Pro Rate batteries any more and stopped doing that about 5 yrs ago, as stated above you just get a new battery (free) with a greatly reduced warranty.
Chad