When you switch the item being smelted by manner of clicking it with another item, so it is never "empty" even for a short amount of time, the smelting progress remains. I know this is a bug, because brewing (which was added later) does reset the brew progress if the ingredient is switched in the same manner.
MC-31203
Furnace Progress Remains After Changing Ingredient
Resolved
Fixed
1
Confirmed
furnace, smelting
Minecraft 1.5.2, Minecraft 1.6.1, Minecraft 1.6.2, Minecraft 1.6.3, Minecraft 1.6.4, ..., Minecraft 13w38a, Minecraft 13w38b, Minecraft 13w38c, Minecraft 1.7.1, Minecraft 1.7.2
Minecraft 14w04a
Retrieved
Attached video