Don’t worry! If it’s a ride you want, hang onto it and then start hacking the night before or the day of and attempt to move the time. This is especially common for the major headliners – the only times left during your initial booking will be later in the evening, and it’s still best practice to grab that.
If once you’re in the parks you can’t modify to a better time, it’s up to you if you’d like to modify it to another ride at an earlier time, so you can then get more. Know that you will never have to cancel a ride though – you can always first go in and try to modify it, and if you don’t change anything, your initial reservation will still be there.
So there’s no harm in trying! It’ll also depend on things like weather, crowds, etc. and how you want your day to look like. Ultimately, it is up to you. We don’t recommend giving up a FastPass for something that you really want to do on your trip, as there are no guarantees.