Inventing a new tag by prefixing it with old_ does not seem a good solution to me. If it changes again you will need older_ or put multiple values in it... it... If the numbers were not reassigned, independent node with addr:* should be fine with some time information like http://wiki.openstreetmap.org/wiki/Proposed_features/temporaryhttp://wiki.openstreetmap.org/wiki/Proposed_features/temporary If the numbers were used again it cannot be really used (there would be two places with the same address)

Inventing a new tag by prefixing it with old_ does not seem a good solution to me. If it changes again you will need older_ or put multiple values in it... If the numbers were not reassigned, independent node with addr:* should be fine with some time information like http://wiki.openstreetmap.org/wiki/Proposed_features/temporary