I think as long as some areas of the map has not been traveled there is a chance for the mobs to be able to still spawn. I myself started my map over for 1.3 since the emeralds but will not be for 1.4 since all the mobs can be spawned in by an admin and same with the new foods. If you have builds you want to save just world edit them to a schematic and copying them to the new map. It's not fun but it will save you some build time.
