Can I replace a cel in a Li-Ion pack with a different cell?
There are several reasons why you shouldn't do this.


The battery pack's controller will probably detect that someone has messed with the batteries and shut the pack down permanently.
The old cells have a way lower capacity than the new one due to aging. This will imbalance the battery pack and cause stress to the balancer circuitry. The controller might also decide that the imbalance is too big for the battery to continue operation and shut itself down.
"Puffing up" is just one step away from "catching fire" with lithium batteries. If one of the cells already puffed up, another one might do this as well and possibly catch fire.
You might damage the cells or battery controller circuit during the repair.
Handling unprotected lithium batteries can be dangerous.