I upgraded my T420 because Ubuntu Mate 19.10 now supports the Nvidia Optimus drivers and includes a utility that lets me switch between Intel and Nvidia graphics cards. However, the upgrade seemed to break the power management on my laptop. When running on the battery, the laptop would suddenly lose power after only 10 minutes, even when the battery still shows ninety percent charge.
I installed Linux Advanced Power Management, TLP. TLP solved my problem. Also, for good measure, I upgraded the BIOS because, while troubleshooting this issue, I discovered is was very out of date.
In his post, I describe how to install and configure TLP and how to upgrade the BIOS on a Lenovo Thinkpad T420.