You can always add/remove journeys.

The advice from the experienced travellers is to never activate the pass until just before boarding the first train of your travel. If you activate your pass in advance and something unexpected happens, like a cancelled train, you can't deactivate the pass after the start date of the validity. 

Also never activate travel days in advance.

Day, khrap. You can even change or add trips on the day itself or when you stand on the platform and see suddenly some other nice destination on the screens. It is as anna tells very UNwise to list them all that long before-unexpected things, either to you or even more from the railways can happen and play havoc with any scheme. (wannee dee kwa proenee).

There are a series of steps in working with the mobile pass that allows you to use your trip as your journey diary where you can build up your travel plans from receiving your pass to your day of travel without fear.

First you add the pass to your app - this does not need any start dates adding or activating at this stage. This will create your TRIP - you can name it. Each pass has only one TRIP for storage of all the trains you intend to travel on. 

This does not activate your pass and until you activate it your TRIP is simply a wish list, and can be modified at any time by adding and removing journeys right up to the point of departure.

Until you activate your pass, ideally on your first day of travel, you cannot transfer any train from your trip to your pass and, even once added, they can be very simply removed from your travel day by moving the yellow slider bar in your trip. You then click create ticket for that day and you will see a QR code for the full day and a growing list of your trains for that day.

Once you move the first train for a day into your pass it will create a TRAVEL DAY. As you travel on further trains that day you move them from your TRIP to your pass and add to the list below the QR code.

So consider your TRIP as simply a birthday diary, and like a birthday diary you can add trains you might travel on, remove trains, travel on a train, not travel on a train in the diary and even add trains right up to boarding. 

How can I cancel my Eurail pass please 

We've already explained to you in a few other topics how to deactivate your pass and where to request a refund. If that doesn't work for you, then please post a follow-up in one of those threads with details of what doesn't work for you.