It appears that the trend is different rules for different airlines. Orion and Monarch (to some extent) pilots flew the originals and classics concurrently, whereas Orion's successor - Britannia - opted to keep the fleets separate. In retrospect this probably made sense as the classics were disposed of after a season or two. I wonder if the original question is still relevant today anywhere in the world ?
FWIW I know at least one skipper who began his career on the NG and only recently began flying the classic as well.