I went from a 52 cm Trek 5500 to a 51 cm Cervelo R3 with no problems. I can't find a geometry chart for the older Trek so it's hard to compare. All of my bikes are set up the same and my boyfriend was able to match the fit on my R3 to the fit I had on my Trek.

I hope that helps. For what it's worth, I don't think you will be disappointed with the R3. I love, love, love mine.