Is it possible to upgrade CPU (ex: from AMD Athlon II X2 to Phenom II X4) without any problems? I means just replacing the old cpu with new cpu at same socket and booting ubuntu with any problems or changing something.
Ubuntu generates most of its hardware configuration dynamically on boot. There are set configurations and overrides (udev stores hardware names so things are consistent between boots, and network configurations can be made to be hardware-specific) but these shouldn't cause terminal issues.
Even the graphics stack is separated out and dynamic these days but you are more likely to run into issues in this area. Again, not terminal issues - just ones that require you to remove the old driver. And that's only in some scenarios.
In short, Ubuntu doesn't care what CPU you're running, as long as it's compatible (which in your case, it is).
