When a character gains a class ability such as an operative exploit, mechanic trick, or stellar revelation where there are various options that only become available at a particular level, can they delay choosing an option for several levels so as to choose a higher level option? For example, could an operative delay choosing their 4th level exploit until they are 6th level, and then choose an extra 6th level exploit?
No.
Thanks!
Could they do this through retraining? In Pathfinder, you only needed to meet prerequisites at the time of retraining, effectively allowing you to ignore legacy prerequisites. Is this true of Starfinder as well?
As an example, could a 6th-level operative have both a 4th-level exploit and a 6th-level exploit, then retrain the former for another of the latter? (Assuming the former wasn't a prerequisite for anything.)
Could they do this through retraining? In Pathfinder, you only needed to meet prerequisites at the time of retraining, effectively allowing you to ignore legacy prerequisites. Is this true of Starfinder as well?
As an example, could a 6th-level operative have both a 4th-level exploit and a 6th-level exploit, then retrain the former for another of the latter? (Assuming the former wasn't a prerequisite for anything.)
I don't think so, at least not without GM approval. Retraining in Starfinder is done with the Mnemonic Editor. I'll go ahead and quote the relevant part:
If you use a mnemonic editor, you can undo 2 character levels’ worth of decisions about which class levels you took, which feats you selected, how you applied any level-based increases to ability scores, how you assigned new skill ranks, and so on. All decisions you made as a result of advancing over the previous 2 character levels you gained are undone. You then make new selections, including new class levels, feats, skills, and the like, as if you had regained the 2 missing character levels. Go through the normal process of advancing your character through each of these 2 levels.
Basically it lets you "restart" the leveling process for the last two levels. If you were level 6 before you used the editor, now you're level 4 with enough XP to gain two levels. Since you still follow the normal process for leveling, you can only pick options that you qualify for at each level.
Gotta pay your dues.
You could, perhaps, with GM approval, not take your powers at a certain level and then take them later when you think you know what you want better (or a situation comes up and "Oh fire resistance might be good right about now).
...but even then, you'd still have to take a power of the appropriate level that you meet the prerequisites for, and you're also stuck with it unless you shell out for a Mnemonic Editor.