For a TBI conversion, you do not have to reprogram the computer but it is highly recommended that you change the chip in the computer, to take into account the sensors you do not use, and maybe also fine-tune the engine to the kind of use the truck will see (race, cruise, economy...). Howell is a place that sells custom-programmed chips.
For an LS engine conversion, you cannot change a chip, you have to have the computer reprogrammed, one of the main reason being the built-in anti-theft system that has to be removed or your conversion will refuse to start (I think it is called VATS).
