Hey there! I totally get your situation - dealing with a 20-year-old PLC that finally gives up the ghost can be a real headache, especially when the replacement has different I/O addressing. Here's my battle-tested strategy that keeps reprogramming to a minimum while ensuring rock-solid reliability:

First, don't panic! The key is creating an I/O mapping layer. Instead of rewriting all your logic, create a translation table that maps your old I/O addresses to the new ones. Think of it as a bilingual dictionary for your PLC - your existing logic speaks the old language, but the new hardware speaks a new one. This mapping layer sits between them, doing all the translation work.

Here's my step-by-step approach:

1. **Document everything first**: Before you touch anything, get a complete backup of your existing program and document every I/O point. This is your safety net.

2. **Create the mapping layer**: Set up a separate program section or use tag aliasing in the new PLC. For example, if your old PLC used I:1/0 for a limit switch, create a tag called 'Limit_Switch_1' that points to the new address like Local:1:I.Data.0. Your existing logic references 'Limit_Switch_1' and never knows the address changed!

3. **Use phased migration**: For larger systems, don't try to do everything at once. Migrate sections of I/O gradually, testing each one thoroughly before moving to the next. This minimizes downtime and risk.

4. **Leverage Rockwell's tools**: Allen-Bradley offers migration tools specifically for this situation. Check if there's a conversion utility for your specific PLC models - these can automate a lot of the address mapping.

5. **Rigorous testing protocol**: Create a comprehensive test plan that validates every I/O point and logic function. Use simulation tools if available, and always test with the actual hardware before going live.

The beauty of this approach is that your core logic stays exactly the same - you're just changing how it connects to the hardware. Once everything's working perfectly, you can gradually optimize the code if needed, but that's a future project, not an emergency requirement.

Remember: zero logic errors comes from methodical planning, not rushing. Take your time with the mapping, test thoroughly, and you'll have a smooth transition to modern hardware without the headache of rewriting decades of proven logic!