I accidentally bought tickets for the wrong day and need to switch to the day after. Do these sights allow these changes?
For Casa Batlló it depends on the type of ticket you bought, check the details on their website. For La Pedrera I'm not sure, but I'm afraid you're stuck. Having said that, if you contact them and explain they might be able to help. In general terms, the Customer Service at most of the most popular sites is quite accommodating -within reason of course!
Correct, I bought separate tickets to each but for the same day. So not allowed at all?
Do you mean you bought entrance tickets to both sites and they're set for the same day? Like you just want to change one of the two? If that's the case, there's no need. You can do both in one day quite easily. And if they're both for the exact same time, you're probably fine - we showed up early for our visits for both of these sites and were let in no problem. The two sites that we visited that were stringent on entry at your exact ticket time were Sagrada Familia and Parc Guell.
I bought a fast pass for Batllo and a regular ticket for Pedrera. Instead of say, Friday, I wanted to go Saturday instead. That's what I mean.
Friday will be less crowded!
and they are only a few blocks apart.... easy to linger in both in one morning OR one afternoon.. though I thought Batllo was more difficult going up and down narrow stairways than Pedrera.
Batllo has a great audio guide- point the screen and get a 360 ofthe room will appear as it once was furnished- nice change from only voices.