Having reread my garbled attempt of an explanation of 74 chips substitutions I would be surprised if anyone could actually follow what I was trying to say   

Imagine the series of chips 74Lxx, 74LSxx, 74xx, 74Sxx

locate the position of the originally specified chip in this string and it can be replaced with any chip that is located to the right.  ie a 74Sxx can be used for anything, a 74xx should not replace 74Sxx, but would be ok for 74LSxx or 74Lxx and so on.   I hope that is a now bit clearer and perhaps useful to some.