By comparing a copy of the r11.4.2 NMFIND.rex file to the version of NMFIND.rex that we ship with AP r11.5 using Notepad++'s Compare plugin, I was able to answer this question. As per the following release notes, some of the programs that we ship, like NMFIND.rex, have had implementation syntax changes so that the code will run under the Open Object REXX interpreter; however, the parameters to NMFIND.rex - the interface - has not changed at all.
You can verify this yourself by opening up your copies of NMFIND.rex and comparing them to the r11.5 version of this same program that I have attached to this case. You can read the comments section at the top of files- compare the parameters' syntax- and you'll see that the interface has not changed at all.
This means that you do not have to change any of your code that calls NMFIND.rex after you upgrade to r11.5