Gradual 105 to Ultegra change - compatibility

[penster]

My roadbike has Shimano 105. Front triple and 8 speed rear with STI shifters. My wheels have 2004 Ultegra hubs.
I want to gradually change over to Ultegra, front triple, 9 (or 10) speed rear, buying the bits off ebay as they appear.
I understand I will need a triple compatible Ultegra front and rear mech. I assume they will work OK with my existing 105 shifters (2004 model).
To convert to 9 speed, do I simply put a 9 speed cassette on the Ultegra rear hub?
When I eventually change the shifters to Ultegra, is there any compatibility issues?
Mick

 

[domtyler]

My understanding is that they are completely interchangeable. Indeed many bikes come new with a mix of the two.

 

[beancounter]

I have 105 on one road bike and ultegra on the other and I'm damned if I can tell the difference...

bc

 

[gavintc]

Are you sure, we are talking 8 speed and 10 speed here? From my limited experience of dealing with 8 speed, it is not compatible with newer versions. I recently upgraded from 9 sp Tiagra to 10 sp 105, there was not to be much left from the old system, so I decided to simply put on a complete new groupset.