Nilutpal,
The change of Windows version is irrelevant. It is still a homogeneous system copy, as long as you aren't changing the entire operating system or entire database platform (you can also change database versions, as long as it's still the same DBMS family (and within some Basis SP and kernel limitations), i.e. you can go from Windows 2003 to Windows 2008 and from MS SQL Server 2005 to SQL Server 2012, and it's still a homogeneous system copy, and yes, the backup/restore method is still valid. You do not need the export/import method for what you're trying to achieve.
Also, SWPM will install a new kernel, but it is not going to change your Basis/NetWeaver or support pack level. You are not installing a vanilla system. You are copying your existing system, including all of its support packs, configuration, etc. You do not install a system on the target host before doing the copy. You simply restore the database, then you run SWPM with the System Copy option. So, YES, you can use the "SAP NetWeaver 7.0 SR 3" option for BACKUP/RESTORE (or export/import, but I'm strongly trying to discourage you from choosing that option) of your sp31 system.
Cheers,
Matt