Hey there! I totally get the pressure you're feeling - inheriting a 15-year-old mixed PLC system is like being handed a ticking time bomb with production on the line. Based on what I've researched, the smartest approach is definitely piecemeal upgrades, not a complete rip-and-replace. Here's why:

First, for your Siemens S7-300 systems, Siemens has excellent migration tools in TIA Portal that can convert your existing logic to run on modern S7-1500 controllers. This lets you keep your existing I/O wiring and field devices while upgrading the brains of the operation. For the Mitsubishi FX series, you can migrate to the iQ-F series (like the FX5U) or iQ-R platforms.

The key advantage of piecemeal upgrades is you can tackle this in phases - maybe start with the most critical or obsolete sections first. You can build new controller racks in parallel, test everything offline, and then execute a single 'swing-over' during a planned maintenance window. This approach typically requires just 1-2 weeks of planned downtime rather than the 6+ months a complete replacement might need.

Plus, with piecemeal upgrades, you're not putting all your eggs in one basket. If something goes wrong in one section, the rest of the line keeps running. You also get to learn as you go and spread the budget out over time.

The bottom line: Take a phased, migration-based approach. It's less risky, more manageable for a junior engineer, and keeps production humming while you modernize. Start with a thorough assessment of your current system, then create a staged migration plan by zone or function. You've got this!