adding on to the other great answers here, packs are made with cells, thousands of cells, arranged in modules... usually the modules are wired in parallel with series connection within the module... to reach the voltage and amperage needed...

you can absolutely swap out modules or even cells if you want, but the cost of doing so is pretty high, usually its easier and cheaper to just swap out whole pack due to the labor cost involved... the swapped out pack can be disassembled at factory and remanufactured into refurbished packs with all the checks and validations needed.

swapping out individual modules and cells comes with the issue of cell imbalance... have you heard of people telling you to not mix new and old batteries in your TV remote? kinda the same thing here but on a larger scale... mixing in new cells with older cells could cause voltage imbalances between the cells due to new cells being able to discharge/charge faster than older cells and also take in or output more power than older cells. this could cause problems, like aging the other modules faster than it would otherwise and/or cause decrease in range.