CYaBro: One of the symptoms of a lithium battery is swelling, if it has reached the end of it's life.
How many times have you charged the battery in those 30 months?
It may just be consumed from your usage, and if that is the case then you're lucky that Apple are replacing it at no cost.
Apple can run a diagnostic that will tell them the number of charges that battery has had, and if it is consumed or a failed battery.

The senior advisor did retrieve the battery stats and claimed everything was normal. AFAIK one of the reasons why Li-Po batteries swell up is because of a failed protection circuit which can lead to overcharging.

StevenWg: As I know, if the battery is swollen and phone is out of 2 years, Apple gives service that paying the battery price to get a replacement device. My friend got the service like this recently.

Apple themselves sugggest the iPhone lasts three years under 'intensive daily use' of the product. Quoting Apple's own environmental report:

"User power consumption assumes a three-year period. Product use scenarios are modeled on data that reflects intensive daily use of the product"

The CGA does not have a hard time limit, and Apple themselves believe their product lasts for three years without major defects. I don't see why defective swollen batteries would not be covered.

Apple's reports can be found here: