Could have = it was possible
Might have = it was permitted

Remember, could is the past tense of can, and might is the past tense of may. Can expresses things that are possible in an objective sense; may expresses things that are permitted or can readily be envisaged (this usually implies that they are objectively possible as well).

* What will you do if you don't become a model? I can become a zoologist. I know that because I have good grades.

* What will you do if you don't become a model? I may become a zoologist. I don't know if it will interest me, though.

Using present and future tenses instead of past tenses makes the statements much more definite, much less hypothetical. Hypotheticals are usually expressed with past tenses (would, should, could, might).