If you've got an SLC 500 running a line, you've probably already had the moment. A card dies, you call around for a spare, and the price makes you sit down. The SLC 500 processors are discontinued (the last of them in 2024), a big chunk of the 1746 I/O is too, and the PLC-5 has been gone since 2017. They'll keep running for years if nothing breaks. The problem is what happens when something does.
First thing I'd do, before planning anything: pull the catalog numbers off every rack and run them through Rockwell's Product Lifecycle Status tool on their website. It'll tell you what's active, what's end-of-life and what's discontinued, part by part. Then back up the program and keep a copy somewhere that isn't the old laptop in the maintenance shop. You'd be amazed how many lines run on a program that only exists inside the PLC.
Then pick how you want to move. There are really three ways to go, and the right one mostly depends on how much downtime you can get.
The phased way. Swap the processor for a CompactLogix and keep the old SLC I/O for now, using Rockwell's 1747-AENTR adapter to talk to the existing racks over EtherNet/IP. The field wiring doesn't move, the outage is short, and you replace I/O later on your own schedule. You're still leaning on old I/O in the meantime, but the brain of the operation is current. Two catches. Check the adapter's supported-module list first; a few specialty and comms cards don't work through it. And the 1747-AENTR is itself End of Life, with Rockwell discontinuing it on February 15, 2027. If you go this route, buy your adapters (and a spare) now and treat it as a short bridge to new I/O, not a forever home.
The conversion-kit way. Rockwell's 1492 wiring conversion systems let you keep the field wires landed on the old removable terminal blocks and connect them to new I/O with pre-made cables. That turns a "re-land every wire" job into an unplug-and-plug-in job, which is a lot less risky when the drawings are, let's say, optimistic.
The full rip-and-replace. New controller, new I/O, re-land everything. It's the cleanest panel when you're done and the cheapest on hardware, but it needs the longest shutdown and good drawings. If the drawings don't match the panel, budget extra time for surprises.
On the software side, you save the RSLogix 500 program as an .SLC file and feed it to Rockwell's RSLogix Project Migrator, which ships with Studio 5000. It spits out a Logix Designer project. That's a big head start, not a finished program: some instructions come across flagged as conversion errors and have to be redone by hand. Messaging, indirect addressing, PID and anything talking to specialty cards deserves a careful look. Plan time to clean up the converted code and to test it before cutover day, not during.
Don't forget the stuff around the PLC. An old PanelView Standard or a DH+ network often has to move at the same time. Rockwell's PanelView Plus migration guides cover bringing older PanelBuilder applications into FactoryTalk View (check the latest one for your software version), but expect to redo screens, and anything that ran over DH+ or Remote I/O will need to move to Ethernet, because the newer terminals don't have those ports.
My two cents: if the line matters, don't wait for the failure to force your hand. A planned migration over a long weekend beats an emergency one on a Tuesday afternoon with production breathing down your neck.