I'm not 100% sure that the conversion only impacts the OTC. Regarding your question about purchasing new shares on the Swiss, you will have to ask someone who is more familiar with the Swiss exchange (I am not). Maybe another poster can respond to that.
The argument 'for the conversion' is that the price movements of the underlying shares on the Swiss will 'track with/to' the ADR shares (and vise versa). I personally do not buy into that notion.
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