The banknotes are fully valid and there is 0 cost to convert them: https://www.shkb.ch/666-austausch-alter-banknoten
I'm P2P maxi since 2019 but yeah, it's difficult where trust is involved.
As mentioned, I can do full KYC, scan my ID, genitals or whatever. From my side, I'd ask for social proof (needs to be industry insider).
BTW - I'm not doing anything illegal. I do legit work and I can fully dox myself.
Bitcoin is fully P2P and it seemed perfectly logical to ask here. If I'd have family or friends in Switzerland, I'd just ask them but I don't have.
If somebody has, please let me know and I'm inviting you for lunch.
No need to complicate further.