Hi
Yea pick new plan, you will have to delete the plan from the start date, it will not delete any of your workouts completed, just new start date.
You can Ramp test any time you feel the efforts are getting too low. The other thing to remember is if New to Ramp test they can be difficult to get used to and you do get better at doing them. Make sure you have had a couple of rest days before attempting, mental readiness is important. Link below shows some tips. The main few are good music, aim to keep power up and cadence and aim to get past 19min 30 seconds.