Yes, you can change unused ones after the time has gone by.
We just got home, and unless you're talking about less popular attractions, don't count on being able to change too easily especially considering a 1 year old's schedule. We noticed TSMM and ETWB in particular running out in the mornings, and we tried changing a PP FP+ sometime in the morning and the soonest we could get was early evening if I remember correctly. Epcot attractions seemed to be sticking around, at least into the afternoons.
We found in order to get times you want for the most popular attractions, which is important with a toddler, it's best to be able to schedule before the parks open. Not impossible to change during the day, but you may end up missing out if you wait.