I trade the ADR/ORD arbitrage for the Mexico/US market and we are able to convert both ways. We use a Mexican broker. It will probably depend on your broker but they should be able to convert both ways. On the retail side I think that Interactive Brokers allows conversions for retail clients; they used to have a minimum $ amount for conversions but that may have changed. There may at times be restrictions on the share movements but for the most part the conversion is fairly straight forward if you have settled shares.