Will Windows 10 Technical Preview be upgradable to final release?
Solution 1:
According to Gabriel Aul, a member of the Windows 10 development team, it is Microsoft's intention to make this possible.
On December 5, someone asked Gabriel Aul the following question on twitter.
will we be able to update to the RTM version when it ships from the TP branch?
Mr. Aul responded with the following response
Yes, that is our intent.
Source