The original battery capacity started to drop, possibly to my ignorance on battery maintenance of a Lithium Ion battery (compared to the old type). So as it got to 95% and dropping I got a new one to place in when it drops to the 80% mark.
The guide is quite easy to understand, I haven't made the switch yet but I'm sure it will go smoothly.
Even though my MBPro is under AppleCare the battery is under it, but they don't replace unless it is under 80% as I understand it. So I'm prepared and shopped around for a good quality replacement.