You want to exchange it after you started travelling? I don’t think that is possible. Buy your pass as soon you know your route.

If you talk about exchanging it before you start travelling, that should be possible, I think the fee is 15%. But be aware, that if you buy the 5d in 1M Pass now with the promotion, you won’t be able to get the 10d pass afterwards with the promotion! I propose, plan a coarse route and decide which pass you take. If you need one day more later, you can just check the normal/saver fare tickets for all the parts to find out which is the cheapest to replace.

Hi, we can't extend or upgrade your existing Interrail Pass with extra travel days. If your travel plans have changed before you started travelling, you can exchange your Interrail Pass for a different pass. It usually costs €15 to make changes to your Pass but if you've purchased Plus, it is free. When you want to add more travel days when you are already travelling, you can order an additional Pass to continue your journey. In this case it's fastest to order a mobile Pass, which you'll receive in your e-mail inbox right away.