Hello i have never heard about rex390 and rex 400
i worked only on rpg/400 ,clp/400,db2/400
Migrate REXX/390 to REXX/400 by copying the REXX/390 source into a source file on an AS/400. The ‘migration’ is then complete.
The REXX is the same on both platforms.
However, you almost certainly have a large number of macro or external procedure references that only exist or even make sense in your mainframe environment. Without those, your REXX will likely be mostly useless.
The REXX itself will be fine. But I wouldn’t expect it to do much that is useful in the ways that you expect or want.
Note that REXX/400 is interpreted — never compiled.