Buying a new original battery will cost over 4 times more than changeing the old cells, as technology grow, new cell will gain more capacity as an example it's possible to use 2500mAh cells insteed of original 2000mAh cells. as a result your refilled battery will bring more working time. If you open a thinkpad battery, you will realize that it was easy possible to design it in a way that users could change the cells but as they profit from selling them, they don't make it that way.
refilling a battery is consist of these tasks:
1) Opening the battery
2) deassembling the cells
3) find and buy new cells
4) reassemble the battery with new cells