One good way to handle this is to add a date/time into the file name.

For instance:

 - instead of **IMG_1234.jpg**, have the file renamed to **IMG_YYYYMMDD_1234.jpg**.  That way if later down the road you have another IMG_1234.jpg, the exported versions would be dated. So if the images were shot on Feb. 14, 2010 and Jun. 30, 2011 you would end up with IMG_20100214_1234.jpg and IMG_20110630_1234.jpg.

In Lightroom, there are [filename templates][1] that can simplify this process.


  [1]: http://help.adobe.com/en_US/Lightroom/3.0/Using/WS436CA8E4-7E7B-4b4c-B522-1449D8F21CE0.html