Hey there! I totally get your concern about what seems like a 'simple replacement' turning into a nightmare. Having worked with these legacy systems, I can tell you there are several hidden landmines that can derail your upgrade project:

1. Hardware Compatibility Nightmares: The biggest shocker is that S7-300 I/O modules are NOT directly compatible with modern S7-1500 systems. You'll need to replace them with ET200SP/MP modules, and here's the kicker - they're different physical sizes! This means you can't just swap them in the same cabinet space, requiring complete rewiring and cabinet modifications.

2. Software Migration Gotchas: You're moving from SIMATIC Manager (Step 7 Classic) to TIA Portal, and it's not just a simple import. Only S7-300 controllers with firmware version 2.6 or later can be migrated. You'll encounter errors like 'interface of DB1 is out of date' and blocks that won't compile properly in the new environment.

3. Firmware and Configuration Issues: That 15-year-old firmware won't play nice with modern hardware. You'll need to run the 'Check Project Consistency' tool in Step 7 Classic first, but even then, hardware configuration problems often surface only during actual commissioning.

4. Hidden Skill Gaps: Your maintenance team knows the old system inside out, but TIA Portal is a completely different beast. The new generation of engineers is trained on modern platforms, creating a knowledge gap that can lead to extended downtime during troubleshooting.

The real danger is thinking this is just a hardware swap. It's actually a complete system migration - hardware, software, and skills. Always backup your original .s7p files, plan for cabinet modifications, and budget for extensive testing and training. What specific modules are you working with? That might help identify more targeted issues!