If I were in your shoes, and I realized that I had to answer a question slightly differently (It should be just slightly!) than I did in the original preliminary hearing, I would just give the most honest answer I could.
Now - there are at lease two schools of thought as to what to say next.
Some will say that you should give the 'new' more accurate answer and then stop talking. Do not say anything about ammending or changing you answer. They will tell you not to open the door to the fact that your answer was different than a previous answer to the same question. If the defense attorney sees the discrepancy, let him ask about it.
Another approach is to answer the question with the 'new' more accurate answer and then go on to say that after the preliminary hearing you had more time to reflect on the issue and realized that your previous answer was somewhat lacking, that this answer was more accurate. The theory here is to difuse the thing before it goes off. Make it a non-issue. The jury (or Judge in a bench trial) will see that you are human, and you are able to correct yourself. This tactic can take the 'wind out of the sail' of the defense attorney and give your testimony an increased amount of credibility.
In any event - I would approach the prosecutor and ask them which approach they recommend (should the issue arise)
Good luck - let us know how it turns out (if you can)
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety."
Benjamin Franklin, 1755
"Living in our society is like living in a very tall building. The rich live on the top floors. The poor live on the lowest floors. And only the cops travel to all the floors. Only the cops see it all."
Det. Ted O'Conner - Chicago P.D