There are additional security issues that arise with 64-bit Office: there are coding changes that will have to be made for the system to run properly in a 64-bit Office environment. There are some coding changes that will make a single front end run properly on both 32 and 64 bit systems.
Here’s a Microsoft article that will help with understanding some of the coding differences requirements that exist between 32 and 64-bit versions of Office (as well as between numbered versions of office–e.g., 2007, 2010, 2013): http://msdn.microsoft.com/en-us/library/gg264421%28v=office.14%29.aspx
There are other changes that have to be made to coding that may cause issues with operation of some parts of the system.
There are a few things: There are some forms (depending on what version you have) that simply will not work in 64-bit. They are built using controls that Microsoft does not support in 64-bit. These include such things as the document tracking screen (it uses the tree view control), the newer tree view menu interface, the report previewer (and associated systems) and some of the help tutorial system. The newer automatic report generator/editor may also not work properly in a 64-bit environment.