Applications that are written in ABL only can be either 32-bit or 64-bit applications, as the r-code is platform-independent.
Applications that use platform-neutral .NET assemblies can also be either 32-bit or 64-bit applications.
Applications which make use of OCXs and/or DLLs need to be reviewed for platform-specific operations. In such cases, if you wish to provide a 64-bit application, you should find 64-bit alternatives for the 32-bit OCXs and DLLs, and change calls to them appropriately.