Yesterday, I was speaking to a bicycle coach who said that the 10sp
Ultegra crankset and BB could be used on a 9sp setup, and vice versa.

Sheldon Brown says this: "Note, also that rear derailers do not care
how many gears you have. You do not need to have a so-called "10-speed"
derailer to use a 10-speed cassette. This is true of all Shimano
derailers with the exception of 1996 and earlier (pre-9-speed) Dura-Ace
units." at http://sheldonbrown.com/k7.html

The same article also says "Also, hubs marked "8-speed, 9-speed or
10-speed will work with any number of sprockets! (Add a 4.5 mm spacer
before installing a 7-speed cassette on an 8-, 9-, or 10-speed hub.)"

However, he then says "2004-2006 Dura-Ace 7800 10 speed hubs only
accept Dura-Ace 10 speed cassettes."

I've heard that apart from the alloys, Ultegra 10sp is mechanically
basically the same as Dura Ace 10, so maybe the same applies to
Ultegra.

Anyway, it does beg the question, what is the absolute minimum change
required to move over from 9sp Ultegra to 10sp Ultegra? Since
various places sell groupset bits one piece at a time, bits could be
replaced piecemeal as they wear out, rather than all at once so I would
have to discard otherwise
perfectly servicable bits.

Maybe I'm reading the various things wrongly, but would I be right in
saying that to convert from 9sp to 10sp I would at minimum only need to
repace the casette, STI shifters and chain? That would suit me fine,
as my STI shifters are getting to the end of their useful life and the
others are considered to be "consumables" anyway.

Travis