Converting between different coordinate reference systems or projection is a fairly standard feature in a GIS. There are a number of command line tools also available for performing bulk-conversions. cs2cs program, part of the PROJ.4 library is my favorite.
Recently, someone presented me with an interesting challenge. They were collecting field data in UTM coordinates and needed a way to convert them to Latitude and Longitude to map them. As the data was being constantly updated, any offline solution that require exporting and importing the conversion would be cumbersome.
You can give it a try. Just open this spreadsheet , make a copy and see the formulae in action. If you wish to change the behavior if the functions, you can edit the code by going to Tools > Script Editor > utmconv.gs in the spreadsheet you copied from above.