Yesterday I have bought online a return ticket (estrella fare) Barcelona-Madrid for a day/business trip on June 4th.
Just hours later I learned that I need to go to Madrid already on the evening of June 3rd. Return from Madrid would be as ticketed.
Trying to change dates online resulted in an error message that my ticket cannot be found in the system.
In the FAQs I did not find any specific regulations re. estrella tickets besides a generic remark that changing/cancelling can be more costly that just the 15% penalty for other tickets.

Questions:
Is it possible to change just the date (now with less than a week to go) of the outbound trip? And if so, why can't the online system find my booking?
And, if not: Would it be possible to buy any other single trip ticket to go to Madrid, but use the return part of the already existing estrella return ticket for the trip back to Barcelona? Or would, as the case with many airlines, the return part of the ticket be void if I don't use the first leg as booked?

As usual, any help will be greatly appreciated.

ay.. anyone?

I have no personal experience, but Maribel claims a 15% fee for a different day for the same day or 20% for another day.

http://maribelsguides.com/mg_renfetickets.pdf

Can you call Renfe? If I remember right, you're in the Munich area, so it should be easy. Good luck.

sorry, I have no answers but once I e-mailed Renfe with a question and got a (helpful) response right away.
(I did write in Spanish)

Many thanks to both of you.
Just out of curiousity I got on the website again and tried the "reimburse" feature instead of the "make changes" button. And miraculously it worked smoothly, found my booking right away, and I was able to return just the first leg of my itinerary.
Now I'll see how I'll get to Madrid instead..