Both versions of TextMesh Pro (Legacy AssetStore & Package Manager) cannon coexist, The AssetStore version is no longer being updated either so it is strongly recommended you upgrade to the Package Manager version which for Unity 2019.3 is version 2.1.0-preview.2. You will need to make sure you follow the directions contained in this thread . The upgrade process isn’t fun to go through but once done successfully, the upgrade process will be much simpler in the future and you will be running the latest releases.
P.S. Preview 3 should be available later today. I am just doing final testing on it.