I have a Portugal short-stay Schengen visa for this Fall for tourism, and in my itinerary I included flying into Lisbon and then flying out of Spain.
If my itinerary were to change such that I was flying both in and out of Spain, but on the same exact dates, would my visa still be valid?
Yes, if you still spend most of the time in Portugal.
You can change the details of your trip after the visa was granted if the premise remains unchanged and if the changes came genuinely after the visa was granted. You should avoid giving the appearance of 'visa-shopping' (that mmeans misrepresenting your itinerary so that another consulate would be responsible for your application).
Your planned change would not affect that you are still goint to Portugal. On arrival in Spain, you would have plans and documents to show that you are immediately going to Portugal, and if asked you can show them.
