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#11
An in place upgrade is possible, at least in the preview version and likely in the final version as well. Windows 10 is compatible with most software that runs on Windows 7. Of course their exceptions. The only way that Microsoft could guarantee absolute 100% compatibility is to make no changes in the OS at all. Obviously that is not an option. Even the most trivial changes in the OS can and has caused compatibility issues Note that an upgrade must be 32 bit to 32 bit or 64 bit to 64 bit. An upgrade from 32 bit to 64 bit or the reverse has never been possible.
A clean install is usually more trouble free but an in place upgrade may be more convenient. Your best option is to make an image backup and do a trial upgrade with 1 computer that has all applications in use. Be sure to do a thorough test with all applications after the upgrade. Depending on how that goes you can decide how to proceed with the others.
Previous versions of Windows have had an upgrade advisor that would warn about most compatibility issues and this will likely be available for Windows 10 as well. You would be well advised to use this before either an upgrade or clean install. This should be available when Windows 10 is released.