Having just finished the walk, what happened to me was blisters on both of my little toes about 3 days before Logrono. I had a stretchy light tennis shoe type shoe to wear, but they hurt the blisters and also, I had nothing to wear for relief in walking when I knew I could go further but my boots were too much on me.
So in Logrono, I went shopping and purchased a 10 euro pair of sandals (that only weighed about 6-8 oz) that were like a heavy foam rubber or something and they were fantastic, not only that, cheap, but the best thing was that I could walk in them in socks and they did not even touch my sore places so I was able to go for many miles in them. they could have been used in the showers but I didn't bother
I would suggest rather than bringing a second pair of something with toes and heels, to definitely get something that has open toes and heels that could be adjusted and worn with socks to walk if any foot problems occur with blisters/sores etc.
here's a photo of mine. Ugly but they worked great, and were soft everywhere yet believe it or not, enough support for about 4-6 hours of walking.
Here is what they were protecting -- I had not even a pain when I wore these instead of my boots.
Good luck!
PS I normally wear Keens or other of that type of sandal & I think if I had brought the ones I normally wear, that they definitely would have rubbed my sore toes, which these very soft ones did not.