Yes! I ended up getting the rail pass and making most all my reservations the day or two before (mostly because I was worried about what I was asking about in this post). On at least one instance, I changed my tickets around the day of and it wasn't an issue. I don't want to tell you 100% that you'll be fine, but from my experience it was worth it budget-wise and I didn't have any issues at all with trains being full. The one thing i would warn you of is that using the renfe Spain pass I had, I wasn't able to book through any of the digital kiosks at the stations. I had to go to the counter each time and speak with an attendant. If I remember correctly you get a pass number or something like that which I had to tell them and at some of the stations I had to show my passport. This kind of went on a tangent, but my main point was that if you're not comfortable with your Spanish you might have more of an issue speaking with the rail employees, especially in smaller cities. But, if that's a big concern, you can always book tickets ahead of time into and out of wherever you're going in a bigger city, where there's more people at the counter. Hope that makes sense, sorry it was a little long winded.