This is covered in the FAQ of the Unity Plus License:
What happens if I upgrade to a higher tier during my commitment period?
If you upgrade during a commitment period, you will have to commit for a new period of at least 12 months. For example, if you choose to upgrade to Unity Pro 6 months into a 12 month subscription to Plus, you will have to start a new 12 month commitment period for Pro. The prior commitment period for Plus will be removed.
So yes, you can upgrade from Plus to Pro at any time. When you do that, your Plus subscription ends (even if still in the commitment period) and a new Pro subscription begins (restarting the commitment period).
Note to future visitors: Back in our time Unity used to change their licensing terms quite often. They might still be doing that in your time. So you might want to verify that the FAQ still says what it said back in March 2019.