Adding/deleting journeys after departure

Is it possible to add/delete journeys from the Pass after the journey has started? And what about after it has finished?

Reason for asking is that I was originally intending to take the Eurostar from St. Pancras to Amsterdam, and I bought a passholder reservation for that journey.

My plans have now changed and I would like to get off the Eurostar early in Brussels. As I understand it this might not technically be allowed but in practice should be OK because there aren’t ticket checks on arrival. I don’t want to change the passholder reservation because there is a charge for doing that.

However, if I change the journey in my Pass to say Brussels instead of Amsterdam it won’t match the passholder reservation. And if I leave it as Amsterdam I can’t add the next train I am going to take from Brussels because it leaves before the Eurostar has arrived in Amsterdam (i.e. it overlaps).

So what I was thinking was to leave the journey in the Pass as Amsterdam until I arrive in Brussels, then delete it and add the same Eurostar train but from London to Brussels instead, which will then allow me to add my onward journey from Brussels.

Will this work? Or is there a better way?

Thanks