Rexx on the mainframe is the original. Anything else is a derivative and many have new functions. One version has the ability to run scripts written for almost any derivative. However, you need the manual for the interpreter version that you are using and you need to skim through it at least once and then read, in depth, those parts that you are going to use now.
There are ways of changing data without using CHANGESTR. You could search the string (POS function) for the string that you want to replace, then split (SUBSTR) the string into before, after and not wanted parts and then reassemble with before, new, after.
You could also do it the way Enrico shows, using using ISPF facilities.