I've spent a long time in that community, and a golden rule is to never mix cells - even cells of the exact same type because they're often binned to be matched to be even closer in operating capacity. Also, and more relevant here, mixing old and new cells is also a big no no due to changing characteristics during aging and use.
You can't just pop open a battery pack and replace a bad cell. I mean, you can, but if you cause a fire that kills someone, you should go to prison for negligence.