Hey there! I totally get your situation - inheriting a 15-year-old production line with mixed PLC systems is like walking into a museum that's still supposed to run a factory! Here's my strategic roadmap for modernization without bringing production to its knees:

First, I'd start with a comprehensive assessment phase. I'd document everything - current hardware, software versions, I/O counts, network architecture, and critical production schedules. This gives me a baseline to work from.

For the Siemens S7-300, the natural migration path is to S7-1500 using Siemens' TIA Portal migration tools. The good news is Siemens provides detailed migration guides that help preserve your existing logic while gaining modern features like better diagnostics, PROFINET support, and future-proof architecture.

For the Mitsubishi FX series, you've got options like the iQ-F series (FX5U) or iQ-R series. Mitsubishi offers transition tools and conversion adapters that can minimize rewiring headaches.

The key is a phased approach: Start with non-critical sections during planned maintenance windows. Use parallel systems where possible - run the new PLC alongside the old one, gradually shifting control. Consider temporary drop-in replacements for critical components to stabilize operations first.

I'd prioritize sections based on risk and production impact, schedule migrations during planned downtimes, and always have rollback plans ready. The goal isn't a big-bang replacement but a carefully orchestrated transition that keeps the line running while future-proofing your operations.