I am looking to begin an upgrade of my Ultegra 9spd drive train and would like to do it slowly, also my front crank is a little worn on the 52 chainring. I was wondering if I got a 6603 10 spd crank, if it would work with the rest of my 9spd gear train. Everything else is still 9 spd. I have heard that the spacing between rings is different and might throw things off. Thanks in advance for any thoughts on this one...

I am looking to begin an upgrade of my Ultegra 9spd drive train and would like to do it slowly, also my front crank is a little worn on the 52 chainring. I was wondering if I got a 6603 10 spd crank, if it would work with the rest of my 9spd gear train. Everything else is still 9 spd. I have heard that the spacing between rings is different and might throw things off. Thanks in advance for any thoughts on this one...
The 6503 is a 30/42/52 and the 6603 will be 30/39/52. I would suggest changing the front derailleur at the same time since the cages are quite different. The spacing dif is 0.4mm. I don't think you will see any difference. If you want, you can put spacers on the ring(s). - TF