Guess its as I phrased it - do you always have to change the eprom if you change from, say, 22 lb injectors to 24 lb injectors? In general is this required regardless of amount of difference?

TIA

 

Well 22 to 24 is almost a 10% increase, so it is significant enough to affect the mixture. If the reason for the larger injectors is because you need more fuel for WOT, then your eprom will be out by 10% during part-throttle.

With an O2 sensor, the ECM can do a fair amount of correction so you probably will get away with it at part-throttle. But where the problem may arise is in "heavy" part-throttle like going up a fairly steep grade. Your rpms may not be high enough to need the extra fuel compensation but your ECM may still go into PE Mode (it only needs a little over 50% on some stock eprom at certain RPMS) at which point you won't have the O2 sensor correcting.

Also, in "start-up" mode, when the ECM is not yet in Closed Loop, you will definitely be running rich which will cause you to burn excessive gas until the engine warms up and gets into Closed Loop.