If I'm a Reform convert to Judaism, will I have to start from the beginning to convert to Orthodox?

Not exactly(Yes and No). You will have to convert. However, your knowledge would be similar to a secular Jew in their eyes etc. Meaning, you would most likely just have to learn the ways of Orthodox Judaism (view of Torah, laws of niddah, how to make a kosher home, keeping shabbat the orthodox way etc). It's not as if you have no knowledge of Judaism and conversion for the most part is long because of learning and practicing and not the rituals involved. It would most likely take no more than 6-9 months and not the 1 to 2 years a person with no knowledge of Judaism would have to do. Also, you would have to dunk in the mikveh and have a meeting with the beit din. But, again, it won't be as long as a normal convert and will focus on Orthodox observance than Judaism as a whole.