Originally the LCD was a bus device. As time has moved on the old hitachi controller is woefully slow for such a task, so we have to 'bit bash' the interface - that is we toggle the LCD signals with code rather than relying on the internal hardware to do it for us.

So the LCD control signals just connect to port pins. See the tutorials section on how to bit bash the LCD interface.

If the software is well written, it is just a matter of changing a few routines to cope with the difference.

The signals you need to take care of are:

E
R/!W
RS
D0..7 (there is also a 4bit mode so you only need 4 data bits)

Depending on what the code expects, you can also not use the R/!W bit and tie it low (write only) as most of the time you only send data to the LCD. The only data usually read from the LCD is the ready status , but you can just use a time delay instead.