Buy cheaper bike and upgrade Can I change group set of this bike from Tiagra to Ultegra 8XXX without changing wheel? If yes, does it depend on geometry of rear fork?

I am asking because I would like to know whether it is cheaper to buy a new bike like this and occasionally change group elements (with used) Ultegra and you will get after some months one pro bike :).

Thanks for your time

More about what I want to buy here
You can do it, but it will be much more expensive to buy the Tiagra-equipped bike and then buy Ultegra parts piece-by-piece. Bike manufacturers get the parts much more cheaply than you can, and that price difference is reflected in the price of the bike.

Also, consider this: Ultegra is an 11-speed group. Tiagra is 10-speed. This means that you will need to upgrade the cassette, rear derailleur, and shifters all at once anyhow.