The IBM i (AS400) began its life as a 48bit machine and was never a 32 bit machine. In fact the S/38 was a 48bit machine. When the RISC processor came out it became a 64bit machine. If you need to compile a program for an older machine you can only go back one release level. Therefore if you are on a V7.3 machine you can only compile back to a V7.2 machine. If you are at a V7.1 machine you can compile back to V6.1. You will find this parameter on the compile option when you prompt the command.
If you need to compile something to a V3R2 (CISC) machine move the source to this machine and compile it on this machine (hopefully you have a compiler on this machine). If you compile on an older operating system and move the object to the new machine it will convert the program from the older machine to the current platform.