From Trojan's website:
Trojan Battery | Battery Maintenance
Can I do a partial replacement of my flooded batteries?
We do not recommend partial battery replacement of flooded battery banks. The behavior of batteries during discharging and charging varies throughout their lifespan; if all the batteries are the same age, they all will have similar responses.
The danger with replacing only one battery is that the older batteries will tend to require more charge than the newer ones, and since the new batteries are in the same circuit, they will also be overcharged. There is also the chance that the older batteries will be undercharged since the overall voltage response of all the batteries (old and new) will not be a good representation of either group. The charging system might erroneously think that ALL the batteries have reached the desired voltage and may stop the charging of the batteries prematurely.
Frank, who used to be the resident guru on all things Golf Cart on this forum, educated me on this a few years ago.
I called (IIRC) Battery Boys and replaced my bad battery with a good used battery of the same age and specs - worked well!
FredF