How hard is it to replace iPhone screen?

Ummmmmmmmm, Which white iPhone? 3G/3GS? And you bought an iPhone 4 to swap the screen back to the 3G? Why not just use the new 4? But to answer your question, it is impossible as the two screens are not interchangeable.

 

You must be a Certified Jedi Night to even attempt to swap iPhone parts between an iPhone 3G/3GS and iPhone 4. The iPhone 4's LCD Screen + Digitizer are bonded together.

 

not to mention that the iphone 3G/3GS does not have the drivers for the screen of the iPhone 4

the software for a certain iPhone is made to work only with that iPhone, and only with the hardware that iPhone has

all the graphics are meant for the lower resolution screen, and the phone wouldn't recognize all those extra pixels, as its programmed to think that its screen is the only possible screen (hooking up to external screens is obviously different), and so it wouldn't know what to do with the new screen

that is, if it is even able to display an image on the new screen at all

 

Its not going to happen.
Dont mess up the unlocked iphone 4.

 

Yup, keep it clean plz.