It's not necessarily easy, but it's definatly possible to learn PLCs from other Manufacturers after learning on the PLC/5.
Most other PLCs (for complex operations anyway) require more bitwise (computer organization) knowledge. This may be something you have to learn as you go. If you have much of a computer programming background, then it should be relativly easy. If this kind of thing interests you, I would advise learning C/C++ at the same time. There are a ton of books and freeware compilers to get you started.
As for the programming position... It really depends. What kind of experience and current position do you have? At my company, a lot of the controls guys got their start doing CAD. It was a good step to learn the drawings and systems and get an introduction to the rest of the engineering staff.