Windows 10 testers should be able to upgrade to RTM version
Members of the Windows Insider program (WIP) who are currently running the Technical Preview version of Windows 10 installed on your PC, Laptop or Tablet. Microsoft has now confirmed their 'intent' to allow users to gain access to the final RTM (Release to Manufacturing) build of the OS when it is released sometime in the second half of 2015. According to Microsoft's Gabriel Aul while answering a recent question on Twitter.
When asked, "will we be able to update to the RTM version when it ships from the TP branch?", Aul's answer was, "Yes, that is our intent." However, there's no definitive word yet on price, if any, WIP users will have to pay to update to the Windows 10 RTM build. There are some persistent rumors that Microsoft will offer Windows 10 for free for users who have Windows 7 or higher installed but the company has yet to comment on pricing for Upgrade or Retail versions of Windows 10.
It should be noted that Aul's statement says that Microsoft's "intent" is to allow Windows Insider user to upgrade Windows 10 to the RTM build. This leaves sufficient some wriggle room for the company to change its mind at some point on this plan - for either commercial or technical reasons.