Dean,
Yes, it is possible to cause a fault of the SLC while tryhing to upload the program from the SLC to your computer. If this is a running machine, I would at least wait until the process is in shut-down mode. If it is not, but simply a program on a SLC that you want to copy, then it is less dangerous. At worst, the SLC could fault, but the program should still be in it, unless you accidentlly download another program on top of it. Just make sure that you always click "Upload" instead of "Download".
If it is a SLC sitting on a shelf for a long time, the program may be lost anyway, if the battery is dead.
When doing an Upload without a previous copy of the program, you do not get any of the documentation (that does not reside inside a SLC 500), but only the bare-bone rung logic. You will have to re-create the symbols and comments yourself. That means you will have to be able to figure out how the program works, otherwise it will not be much use to you. An old print-out always helps, and if that is not available, an old connection drawing is next best, third best is the machine itself, with I/O wires labeled, pushbuttons and switches identified.
