Good news for all the folks who took risk and upgrade their primary machine to Windows 10 Technical Preview / Insider Preview for Desktops & PC. Head of Windows Insiders Program, Gabriel Aul, has confirmed on twitter that all the users will be able to upgrade to the final version a.k.a RTM build of Windows 10 for free.
While we are expecting that RTM version should be made available around June, this is not final confirmation from Microsoft, but as seen on one of the slide show released to OEM to get their PC ready.
— Gabriel Aul (@GabeAul) May 10, 2015
That said, I have tried Windows 10 Preview from a long time, and the current build is pretty stable so may be you can install it or upgrade your home machines, but keep it off your home machines for now. Microsoft has said that Windows 10 will be free upgrade, which will be valid for a year, and that would be the right time to upgrade any of your work PC.
Via Gabe Aul