In 1988 authorities discovered that gravity affected the measurements of heights (which had been based on 1929 principles). The results mean that big mountains such as the Rockies are as much as 6-7 feet higher (or more) and lower lying areas could actually be a foot or so shorter. The USGS has not fully adopted it because the cost would be billions to readjust all the maps. Plus the logarithm is complicated and does not fit into a traditional Excel style formula. The 1988 data is known as North American Vertical Datum of 1988. The 1929 data is known as the National Geodetic Vertical Datum of 1929.
Download Vertcon 2.1 from the National Geodetic Survey (NGS) for a Windows command line program that will do the conversion for the Lower 48 (formulas are not yet available for Alaska and Hawaii). Although intimidating in the readme.txt, it is fairly straight forward. Downloading and installing the zipped archive is the simplest way for this work.
You can also calculate heights online with a simple version of the program.
Read our article on the isues.