Can I replace just one battery in a string?

Technically yes, practically it is usually a false economy, and there is a real risk in doing it badly.

In a series string, every battery carries the same current. A new battery has lower internal resistance and higher capacity than its aged neighbours, so on discharge the older units reach their end voltage first and the new one is never fully exercised; on charge, the string voltage divides unevenly. The result is that the new battery is dragged toward the condition of the old ones, and the string’s capacity is still set by its weakest member. You have spent money without buying runtime.

A weak cell in a string can be over-charged by the float voltage intended for the string as a whole. Sustained over-charging is a recognised thermal runaway mechanism in VRLA batteries, and a failure mode observed across utility fleets managing critical infrastructure.

Sensible practice:

• Within the first year of installation, replacing a single failed unit is reasonable, the rest of the bank is barely aged.
• After roughly a third of design life, replace the string.
• Never mix chemistries or design-life classes in a string.
• Never mix lithium and VRLA on the same bus without separate charge control. Their float and charge acceptance characteristics are incompatible, and the charger cannot serve both correctly.