Hey there! I totally get your situation - inheriting a 15-year-old production line with mixed PLC brands is like walking into a tech museum that still needs to run production! The good news is you don't need months of downtime to modernize. Here's a pragmatic phased approach:

1. Start with a thorough assessment - Document everything first. Map out what each PLC controls, their criticality, and which ones are most problematic. Siemens S7-300 is actually being phased out starting October 2025, so that might be your priority.

2. Use a parallel migration strategy - Install new PLCs alongside the old ones. This lets you test the new system while the old one keeps running production. You can switch over during scheduled maintenance windows or planned downtime.

3. Consider a phased approach by section - Modernize one machine or production section at a time rather than the whole line. This spreads the risk and keeps most of your line operational.

4. Standardize on one platform - Pick one modern PLC platform (like Siemens S7-1500 or Allen-Bradley ControlLogix) that can handle all your needs. This simplifies future maintenance and training.

5. Add a supervisory layer - Implement an OPC UA server or SCADA system that can communicate with all your PLCs. This gives you better visibility and control while you're in transition.

The key is to avoid the 'big bang' approach. With careful planning and a phased strategy, you can modernize without shutting down production for months. Start with the most critical or problematic PLCs first, and build momentum from there!