You printed AreaList, which doesn't get changed even though you would be printing and changing the values as you went.
>>> AreaList = ["1000 feet", "23908 feet", "1200000000 feet"]
>>> a = [i.replace('feet','meters') for i in AreaList]
>>> a
['1000 meters', '23908 meters', '1200000000 meters']
>>>
Now see what went wrong.
>>> for i in AreaList:
... i = i.replace('feet', 'meters')
... print("i value {}".format(i))
...
i value 1000 meters
i value 23908 meters
i value 1200000000 meters
>>> print("Area list at the end \n{}".format(AreaList))
Area list at the end
['1000 feet', '23908 feet', '1200000000 feet']
>>>
Got to watch what you think is going on... just because a value is pulled from and list and changed, doesn't mean the value in the original list gets changed. You have to recollect the changes as I did in the original example, or replace them in the input list.