[Dominik]

Copper work hardens during operation (vibration) and will crack. It's a no-go for roadworthy (every 2 years) in Germany. There seems to be copper version preventing this, but why bother?

I could also check the homologation papers, but they are 6.000 mls away,

If your concern is value of the car and that's your motivation to convert it back, I wouldn't.
At all, ever. Takes too much effort and time. Any capable shop is booked two years in advance.

Drive it.

Get a quote for the conversion from a well known shop, say $50.000. And it's worth now - $950.000? That's the negotiation space anyway. If you buy/sell it. 5%...

If you invest for the conversion you will get negotiated down back to where it was before.
Unless you are a good salesman, which I am not...:-)