When replacing the modular Chassis for a SLC 500, will I need to reprogram everything, or just place all of the existing modules back into the new chassis and fire it up?
If you are talking about the backplane into which all the boards snap into, no reprogramming will be necessary PROVIDED:
i) your program is saved in a processor with a good battery, and
ii) you plug the boards back in in the same spot they came out of.
The SLC500 has a configuration window in which you have to select all components used. This includes the backplane. If you move from a 7 slot to a 10- or 13? you have to "tell' the processor.
In general the program is stored in CPU and either ram or prom so you should be able to just take everything out and put it back in the new chassis after you're done. Having said that, it's also possible that the battery is dead so you probably want to check that first before you turn the power off.
Did you mean you are going to replace only the chassis with same size and use the existing for the rest ?
If so, the answer is yes, no need to reprogram as long as the existing program stored on the CPU module is not lost during CPU module moving from old chassis to new chassis, means backup battery should be on good condition.