I think you could just use a regular fill in the blank. Then later on, present another fill in the blank and insert the prior item's variable name in the question wording. If the anagram is tca and the first time they answer, the variable name is q17 and they said "act", then the next time you could use the question wording...

Previously, when you saw the anagram "tca", you replied <q17>. Would you like to change your answer?

__________
PS. Sorry I misread and did this example with anagrams instead of antonyms but it's the same idea..