Page 29-8 of the 48GX User's Guide says that to save changes to a program, simply press ENTER after the changes are made. I do this but find later that the changes didn't take. What am I possibly not doing correctly? Thanks.

The Enter saves the changes in the stack but does not store the program into the memory under whatever name you may have given it. You have to replace the existing program with the modified program through an additional store command (or as a short cut DEF key and program name in VAR menu).

After you have made changes to you program and have pressed enter. It changes it on the stack. But to change your program into memory, you press the purple arrow button and the appropriate white menu button that shows your program name.