Does it really diffrent if you install ubuntu mate fresh or migrate from ubuntu via tasksel?
I have used both in the past.
I would high recommend doing a fresh install of Ubuntu mate or any O/S. So you don't get conflicts or errors.
I have found that tasksel to be problematic and not the full install.
You can install ubuntu-mate-desktop and then uninstall the desktop environment that came with your Ubuntu flavour. But the prebundled software that came with your Ubuntu flavour will most likely stay on your system. The only true way to migrate to another Ubuntu flavour is to do a clean install of that flavour.