hi, im looking at a giant ocr composite frame bike but the only one i can afford is the tiagra model.

is it possible to upgrade some of the tiagra components later on with 105/ultegra??
perhaps the shifters or derailuers if possible but tiagra is 9 speed and 105/ultegra 10 speed, so are they compatible??

there's plenty of cheap shimano stuff on ebay i could buy for upgrading.

thanks.