In Reply To:
....
But there will be some difference in the operation, as the 10-speed front deraill. is designed to manipulate a much narrower chain (the quite narrow 10-speed chain). If you can afford getting the 10-speed front deraill. now, it will be more responsive in the way it shifts (and it is also a few grams lighter).
Another complicating factor: if you use the 9-speed front deraill. and later don't like how it works and go to the 10-speed one later, you will have to break (disassemble) the chain to change front deraill's. I haven't does this with a 10-speed chain, but I have read in a few places that DA chains are expensive, not easy to disassemble, not easy at all to reasemble correctly, and very easy to mess up.
....
The 10-speed chain connecting pins are 0.6mm shorter than the 9-speed ones, and the difference in chain width is roughly 1 mm at the widest place. Not that "much" in absolute terms.
The 10-speed chains do see to be a tiny bit harder to assemble correctly than the 9-speed ones, but why should they be hard to disassemble (a total non-brainer)? To be on the safe side, I would simply make the chain perhaps one link longer than absolutely necessary (cutting and reassembling the chain costs one link, if you cut it in the same place you assembled it).
All this said, the front derailleur isn't that expensive, so it might make sense to exchange it anyway (but, as I've already said, I've had no problems with the 9-speed). version
Simon