So we had virtually no line ups and we thoroughly enjoyed the short tour of the impressive building.
If this was a ticket office, you'll need to complete a refund application form.
If this was over the telephone or the internet, you should return it by post with a covering letter.
This was the response directly from Palacio da Bolsa: Even if you buy the tickets on line, you still must come to our ticket office to see what tours are available that moment, do the line and book your place in one of them (guided tours are mandatory)!
So I doubt they would accept visitor tours too far in advance anyway.
We went to the building and bought tickets for an English language tour a couple of hours later.
Check at the station before you travel: you may find you don't need to pay an additional fare as long as your outward travel date remains the same.