Hey there! I totally get where you're coming from - you've inherited this aging production line that's still chugging along, but you know those S7-300 PLCs are getting long in the tooth. The smarter play is definitely strategic component-by-component migration, and here's why:

First, let's address the 'it still works' mentality. Yes, it works today, but Siemens is phasing out S7-300 support (some sources mention 2025), which means parts are getting harder to find and more expensive. When one fails, you could face weeks of downtime waiting for a replacement.

For ROI justification to finance, focus on these key points:

1. **Risk Mitigation**: A strategic migration reduces production risk. Calculate the cost of one day's downtime versus the migration cost - that's your safety net.

2. **Maintenance Savings**: New S7-1500 systems can reduce maintenance costs by up to 40% and cut unplanned downtime by 85% through predictive maintenance capabilities.

3. **Energy Efficiency**: Modern PLCs are more energy efficient - show them the electricity savings over 3-5 years.

4. **Future-Proofing**: The TIA Portal platform for S7-1500 allows easier integration with modern systems and better cybersecurity - essential for today's connected factories.

5. **Phased Investment**: Component-by-component migration spreads costs over time. Start with the most critical or failure-prone components first, using planned maintenance windows to minimize disruption.

The complete overhaul approach is tempting but risky - it's like replacing your car's entire engine when you could just upgrade the most worn parts. With strategic migration, you keep production running while systematically improving reliability. Present it as insurance against catastrophic failure rather than just an upgrade expense!