Hey there! I totally get the dilemma you're facing with that Mitsubishi Q-series PLC. It's like being stuck between a rock and a hard place, right? Let me break down how I'd approach this tricky situation.

First, let's talk about the NOS (new old stock) option. The good news is that Mitsubishi Q-series PLCs are still available as NOS from various suppliers. This gives you a direct replacement without any programming changes or retraining. The hidden costs here are mainly availability - you're buying time-limited inventory that will eventually run out completely. Plus, you're just postponing the inevitable replacement.

Now for the upgrade path - this is where those hidden costs really add up. You've got to consider:

1. Programming costs: Converting your existing Q-series logic to a newer platform isn't just copy-paste. You'll need engineering hours for testing and validation.
2. Retraining: Your maintenance team and operators need to learn the new system. This means training time, potential mistakes during the learning curve, and maybe even hiring temporary help.
3. Production downtime: This is the big one! The average cost of downtime can be $260,000 per hour across businesses. Even a few hours of planned downtime for the switchover can be incredibly expensive.
4. Integration issues: Newer PLCs might not play nicely with your existing equipment, requiring additional hardware changes.

My approach would be to create a simple cost-benefit analysis. Calculate the total cost of NOS (including future replacements) versus the one-time investment in upgrading. Don't forget to factor in that Mitsubishi has announced end-of-life dates for Q-series, with service ending around 2033. So NOS is really just a temporary fix.

What's your current production schedule like? Are you in a position to handle downtime, or is every minute of uptime critical?