You should replace some functions with their new versions when porting an application to 64-bit systems. Otherwise, the 64-bit application might work incorrectly. The analyzer warns about the use of deprecated functions in code and offers versions to replace them.
Let's consider several examples of deprecated functions:
Extract from MSDN: To control whether a 64-bit application enumerates 32-bit modules, 64-bit modules, or both types of modules, use the EnumProcessModulesEx function.
Extract from MSDN: This function has been superseded by the SetWindowLongPtr function. To write code that is compatible with both 32-bit and 64-bit versions of Windows, use the SetWindowLongPtr function.
Extract from MSDN: When lpFileSizeHigh is NULL, the results returned for large files are ambiguous, and you will not be able to determine the actual size of the file. It is recommended that you use GetFileSizeEx instead.
Be careful, if you want to replace the 'lstrlen' functin with 'strlen'. The 'lstrlen' function cannot correctly evaluate the length of the string if this string contains of more than 'INT_MAX' characters. However, in practice the possibility to see such long strings is really low. But as opposed to the 'strlen' function, the 'Istrlen' function correctly processes the situation when is is passed a null pointer: "If lpString is NULL, the function returns 0".
If we just replace 'lstrlen' with 'strlen', then the program can start working incorrectly. That's why usually it's not recommended to replace 'Istrlen' with some other function call.