The term Win64 is used in various contexts.
When one speaks about a Win64 application, you should understand that one means a 64-bit program to be executed in the 64-bit environment and that has the LLP64 data model. A Win64-program cannot be launched in the 32-bit Windows version.
By Win64 a 64-bit API is also understood which is provided by the 64-bit Windows operation system.
There is a subsystem WoW64 on new 64-bit Windows systems to provide compatibility with obsolete 32-bit applications. The system translates the calls of the 32-bit API into the calls of the 64-bit API, redirects requests when working with the register and files and performs some other functions.
Usually a Win32 program can be recompiled in the 64-bit version after some slight modifications. The project fragments written in inline assembler may appear the most troublesome at the stage of compilation. It is very important to provide a search of hidden defects in the code after building the 64-bit version. To detect such defects you may use the tool Viva64 included into PVS-Studio we are developing.