Happy Ending After A Defective Replacement Battery From Amazon
The battery in my five and a half year old Mac Book Pro swelled up and distorted the case.
My first step was to remove the old battery to relieve the pressure on the case. I then ordered a cheap replacement from Amazon. It arrived and I installed it. The computer booted up and then kept rebooting over and over. At this point I did what I should have done in the first place and ordered a battery from iFixit. Due to time pressures and my proximity to San Luis Obispo, I drove up to iFixit and picked up the battery the same day. Installed the iFixit battery and everything is working well again. Got full refund from Amazon seller.
As a footnote, I already had the necessary drivers to remove the battery. If you don't have these, you will need them.
Avoid cheap aftermarket batteries. The batteries have a chip which lets the computer recognize the battery. The chip in the Amazon battery was clearly either defective or the incorrect version. I could have saved a lot of time and stress by just ordering from iFixit in the first place.
The new battery seems to last very well.