You can respec any time between missions, or just buy more mastery boards to store multiple builds and switch betwen them as desired. PROTIP though, if you're respeccing a lot, don't clear your entire board every time unless you really want to change the entire build. Just remove and replace what you no longer need and leave the rest alone. You get a shitload of free training manuals, but if you're constantly redoing your builds you're going to run out sooner or later and it'll start digging into your otherwise quite generous cash supply pretty fast.
Story decisions can always be redone as well, by simply replaying the mission. This is useful in a few cases where different decisions lead to different rewards or unlock different scenes for the gallery. You're never locked out of anything since every mission can be replayed.
The only thing you can kind of get locked out of is rewards from the sidequests the shopkeepers give you, since you can only do those once, but there are always other ways to get those rewards (by crafting or farming) so it's not a big deal.