Hey there! As an engineer facing this exact challenge, I totally get where you're coming from. You've got reliable old Modbus RTU devices that just keep working, but you need to bring them into the modern OPC UA world without disrupting production. The good news is there are practical solutions that let you bridge this gap without falling into 'digital transformation paralysis.'

Here's a realistic approach that works:

1. **Use protocol gateways/bridges**: Devices like the Anybus X-gateway IIoT or Red Lion Data Station Plus act as translators between Modbus RTU and OPC UA. They sit between your legacy devices and modern systems, converting data in real-time without touching your existing production setup.

2. **Start with a pilot area**: Don't try to upgrade everything at once. Pick one production line or area where you can test the integration. This minimizes risk and lets you work out kinks before scaling up.

3. **Maintain dual operation**: Keep your existing SCADA/control systems running while you gradually add OPC UA connectivity. This ensures production continuity - if something goes wrong with the new setup, you can fall back to the proven system.

4. **Focus on data mapping first**: Before any hardware changes, map out exactly what data you need from your Modbus devices and how it should appear in OPC UA. This planning phase is crucial for smooth implementation.

5. **Consider software platforms**: Tools like Ignition or Kepware can bridge legacy devices to modern architectures, often with less hardware investment than you might think.

The key is to think of this as an evolution, not a revolution. You're not replacing your reliable Modbus devices - you're giving them a new way to communicate. Start small, prove the concept, and scale gradually. This approach keeps production running while steadily moving toward your modern OPC UA architecture goals.