For example, if I have an old p4 acer laptop with a perfect screen, and a newer dual core acer laptop with a cracked screen, can I swap the screen in to the newer unit (assuming it's the same size)? Or, do they change the cables/interface technology so they are not compatible?
Yes you can, but...
Look at this picture, and the connector on it. It is pretty common connector on newer model notebooks... after Celeron ones. Any really older notebooks have varying, and propriety connectors that will not be compatible.
I think these are called PPI but many distributors call them
That type of connector works with any size LCD, so you can use a 13" in 15.6"- obviosly it wont fit in the frame it was built for.. but the picture will work fine, because it uses the same protocol and image processor on the LCD chip.. So it can handle the scaling, refresh rates and all that mumbo jumbo directly on the LCD, making it an easy standard to work with.
This is how the inverter looks like...
Notice this cable has golden fingers, not pins.
Here are some old connectors are are not cross compatible
Then you also get this weird PCB, that is exactly the same as the cable in the first picture (PPI), it is the same connector, and these are usually found in Dell's and Acers, sometimes a WEBcam connector comes off it too.