Mobile pass that hasn’t been used can be deactivated and reactivated for the day you wish.

There is no good reason to activate pass and travel days far in advance, once a day has begun (midnight CET) you can no longer deactivate it so only add days you are 100% sure you are going to use.

Regarding the reservation, have you read these sections of the interrail website?

https://www.interrail.eu/en/support/reservations/can-i-get-a-refund-for-a-reservation-ive-purchased-but-wont-use

https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy

It is likely your only option will be to apply for a partial refund (if offered) and book a new reservation so you should check to see if passholder spaces are still available on the train you now wish to use.

What about taking a train that has already been fully booked out?

You can’t on Eurostar.

Or you can try to find a stop before or after your destination, and see if there’s still availability for that.

When and where are you going? The more details of the train you’re taking/want to take, the better we can help.

You can change a Eurostar reservation on the Eurostar website, via Manage Booking. Also if you booked via Interrail. The change fee is €15.

Thank you!

Is it possible to change a reservation onto a train that is no longer bookable for passholder though?

If so it would be a sneaky way to dodge the passholder limitations for everyone.

It is one of those things that is not able to test book without an actual E* reservation and as I haven’t used the tunnel for several years...

I wouldn't expect so.